Plastic foam material composed of thermoplastic resin and silane-modified thermoplastic resin and method for making same

ABSTRACT

A plastic foam material composed of a blended resin composition which includes at least two thermoplastic resins and a silane-modified based resin. The blended resin composition consists essentially of 100 parts by weight of at least two thermoplastic resins, from about 1 to about 50 parts by weight of a silane-modified, cross-linked, thermoplastic resin; from about 0.001 to about 2.5 parts by weight of a cross-linking catalyst for use in a silane compound and from about 1 to about 20 parts by weight of a foaming agent. According to one embodiment of the present invention, the thermoplastic resins include at least two substantially incompatible and substantially uncross-linked thermoplastic resins, while in another embodiment the two thermoplastic resins need not be substantially incompatible. The blended resin composition may further include reinforcers such as glass fibers and filler. The thermodecomposition foaming agent decomposes at sufficiently high temperatures to yield a plastic foam material. The resultant plastic foam material, in turn, exhibits superior resilience, improved compression strength and superior heat-insulating properties. A method to produce the plastic foam material from foamable tubes and foamable pieces is also disclosed.

This is a divisional of application Ser. No. 08/309,960, filed Sep. 21,1994, now U.S. Pat. No. 5,552,448.

BACKGROUND OF THE INVENTION

This invention relates to a plastic foam material. More specifically,this invention relates to a plastic foam material composed of a blendedresin composition containing thermoplastic resins and a silane-modifiedresin (hereinafter referred to as "plastic foam material") and productsderived therefrom. The present invention is also directed to a method ofmaking the plastic foam material.

The present invention is also directed to a plastic foam material whichhas resin walls which include lattice-shaped or honeycomb-shapedcross-sections with foam inside. The present invention also relates to amethod for making the same.

The plastic foam material of the present invention and products derivedtherefrom exhibit improved softness and superior heat-insulatingproperties. They are thus capable of being used as heat-insulatingmaterials. Because of their softness, superior heat-insulatingproperties and lightness, products derived from the plastic foammaterial of the present invention, can be used extensively in manyapplications in different fields.

Products composed of the plastic foam material of the present inventionare well suited for use as building materials. Such building materialsinclude ceilings and installment panels. Due to their superiorheat-insulating properties, the plastic foam material will findwidespread use as aircraft, train and automobile components such aspanels and seating. Various plastic foam material composed of resincompositions are commercially available but almost all of them areinferior in either heat resistance or foam generation. Most prior resincompositions which form prior art plastic foam materials exhibitexcessive gel fractions and inner stresses imparting inferior qualitiesto conventional plastic foam materials and their derived products.

Notwithstanding increased heat resistance properties of conventionalplastic foam materials, prior art plastic foam materials are well knownfor generating less foam and yielding foam materials that haveinadequate compression strength. Accordingly, conventional plastic foammaterials and their derived products are not suited for use insidebuildings, as building materials or as materials for use insideautomobiles, trains, and aircraft.

Many attempts have been proposed to overcome the aforementioneddrawbacks. Unfortunately, to date the proposed improvements have beeninsufficient.

One such attempt at improving the overall quality of prior art plasticfoam materials, disclosed in Japanese Laid Open Patent Publication S58-134131, includes using a cross-linked polypropylene based resincomposition as the starting material. The cross-linked polypropylenebased resin composition further includes a silane-modified polypropylenebased resin, together with a silanol condensation catalyst and a foamingagent.

However, the proposed resin composition is plagued by numerousdrawbacks. Chief among them is the even cross-linking among the variousconstituents when a cross-linking agent is added to the startingmaterial. Since the starting material includes a polypropylene basedresin composition which is thermally grafted by an ethylene typeunsaturated silane compound, the entire plastic foam material is evenlycross-linked upon addition of a cross-linking agent. This, in turn,increases the inner stresses within the plastic foam material,particularly upon heating.

Moreover, the amount of the silane-modified resin added to the proposedmixture is excessive compared to the other resin. The excessivesilane-modified resin results in even cross-linking among the variousconstituents, resulting in an increase in the gel fraction of theresulting plastic foam material. The increased gel fraction of theplastic foam material, in turn, causes a subsequent decrease in themoldability of the plastic foam material. The decrease in themoldability properties of the plastic foam material, in turn, results inarticles which are considerably weak and easily breakable. The derivedproducts are unable to maintain and retain their shapes, due, in part,to the compromised moldability of the plastic foam material.

In an attempt to improve the compression strength of conventionalplastic foam material and products derived therefrom, Japanese Laid-OpenPublication No. S52-104574 discloses a method for making a foam compoundthat uses two extruders, where one extruder extrudes a plasticcontaining a foaming agent to form a core, while the other extruderextrudes plastic to cover the thus formed core. This is injected into ametal mold and foamed.

However, because this method involves the injection into a metal mold ofa plastic containing a foaming agent and a separate covering plastic, itis difficult to provide a uniform feed of the plastic containing thefoaming agent.

Thus, it is impossible to form a plastic column that penetrates both thefront and back of the resulting plastic foam material. It is even lesspossible to form a plastic column uniformly in the plastic foammaterial. Thus, the compression strength of the resulting plastic foammaterial is inadequate.

On the other hand, honeycomb structures are porous and have highcompression strength. A honeycomb structure involves a honeycomb formsandwiched between surface materials.

Notwithstanding the presence of the honeycomb structure which increasescompression strength, conventional plastic foam materials according tothis publication exhibit increased heat conductivity. This is because ofthe convection which results from an increase in internal space. Thehoneycomb structure thus imparts inadequate insulating properties to theresulting plastic foam material.

Japanese Laid Open Publication No. 4-151238 also attempts to improve thecompression strength of conventional plastic foam materials by using aresin foam material connected to a fiber-reinforced resin layer by aplurality of columns. The space between the layers is filled with foam,except for the space occupied by the resin columns.

However, the reinforcing effect of this resin foam material isineffective because resin columns are embedded in foam and thus are notcontinuous. Additionally, to effectively increase the compressionstrength of the resin foam material, it is deemed necessary to embed alarge number of columns, making the resin foam material substantiallyheavy.

In an attempt to overcome the aforementioned deficiency, JapaneseLaid-Open Patent Publication S61-59339 discloses a plastic foam materialcomposed of a resin composition containing a copolymer consisting ofethylene and an unsaturated silane, a silanol condensation catalyst anda foaming agent.

According to this publication, a copolymer containing at least ethyleneand unsaturated silane compound, among others, can be used instead ofthe copolymer containing an ethylene like olefin resin and theunsaturated silane copolymer.

However, the proposed plastic foam material composed of theaforementioned resin composition exhibits a high gel fraction andincreased thermal deformation. The gel fraction has been reported to besubstantially high. The increase in gel fraction and thermal deformationmakes such a plastic foam material unsuitable for use in molding theproposed plastic foam material into large objects requiring increasedstability.

Japanese Laid Open Patent Publication S56-109229 proposes the use of aplastic foam material composed of a silyl-modified ethylene basedpolymer together with an ethylene based polymer, and a foaming agent.

The proposed resin composition contains a substantial amount of asilane-modified ethylene based polymer per 100 parts by weight of theethylene based polymer.

However, articles made of the disclosed plastic foam material exhibitincreased inner stress. The inner stress, in turn, causes extensivethermal deformation.

None of the prior art references manage to overcome the problem of thehigh gel fractions and inner stresses.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a plastic foam material andits derived products which overcome the deficiencies associated withprior art plastic foam materials.

It is a further object of this invention to provide a method of makingthe plastic foam material.

It is still a further object of this invention to provide a plastic foammaterial which exhibits superior formability in thermoforming andforming products therefrom.

It is yet another object of this invention to provide a resincomposition which exhibits minimal thermal deformation when forming aplastic foam material.

It is also a further object of this invention to provide plastic foammaterial composed of a resin composition which exhibits superiordimensional stability.

It is still a further object of this invention to provide plastic foammaterial which exhibits superior heat resistance.

It is still a further object of this invention to provide a plastic foammaterial composed of at least two thermoplastic resins which exhibitsimproved elongation at high temperature.

It is still a further object of this invention to provide a plastic foammaterial composed of thermoplastic resins which exhibits superiorsecondary processing properties when it forms a layered structure inconjunction with a substrate.

It is still a further object of this invention to provide a method forproducing plastic foam material which is light and exhibits improvedheat-insulation properties.

It is still a further object of this invention to provide a method forproducing a plastic foam material which has high compression strengthand good resilience.

Briefly stated, the present invention provides a plastic foam materialcomposed of a blended resin composition that includes thermoplasticresins and a silane-modified thermoplastic resin. The blended resincomposition includes 100 parts by weight of at least two thermoplasticresins, from about 1 to about 50 parts by weight of a silane-modifiedthermoplastic resin; from about 0.001 to about 2.5 parts by weight of across-linking catalyst and from about 1 to about 20 parts by weight of athermodecomposition foaming agent.

According to a first embodiment of the invention, the two thermoplasticresins are substantially incompatible with each other (hereinafterreferred to as "two incompatible thermoplastic resins"), while in thesecond embodiment, the two thermoplastic resins need not be incompatible(hereinafter referred to as "two thermoplastic resins").

The blended resin composition may further include specified amounts ofreinforcers such as glass fibers and fillers.

The thermodecomposition foaming agent decomposes at sufficiently hightemperatures to yield a plastic foam material. The resulting plasticfoam material, in turn, exhibits superior resilience, improvedcompression strength and superior heat-insulating properties.

According to an embodiment of the present invention, there is provided aplastic foam material which includes 100 parts by weight of a at leasttwo substantially incompatible thermoplastic resins selected from thegroup consisting of a polyethylene, polypropylene, ethylene-propylenecopolymer, ethylene-vinyl acetate copolymer, polystyrene and similarresins.

The plastic foam material further includes from about 1 to about 50parts by weight of a silane-modified thermoplastic resin of the samepolymer type as at least one of said two incompatible resins, togetherwith from about 0.001 to about 2.5 parts by weight of a cross-linkingcatalyst effective to cross-link the silane-modified thermoplasticresin, and from about 1 to about 20 parts by weight of a foaming agent.

According to another feature of the present invention, there is provideda plastic foam material which includes 100 parts by weight of at leasttwo thermoplastic resins selected from the group consisting of apolyethylene, polypropylene, ethylene-propylene copolymer and similarresins.

The plastic foam material further includes from about 1 to about 50parts by weight of a silane-modified thermoplastic resin of the samepolymer type as at least one of the two thermoplastic resins togetherwith from about 0.001 to about 2.5 parts by weight of a cross-linkingcatalyst effective to cross-link said silane-modified thermoplasticresin, and from about 1 to about 20 parts by weight of a foaming agent.

According to another feature of the present invention, there is provideda method for preparing a plastic foam material comprising thermoplasticresins, which includes preparing a first mixture containing 100 parts byweight of at least two substantially incompatible thermoplastic resinsselected from the group consisting of polyethylene, polypropylene,ethylene-propylene copolymer, ethylene-vinyl acetate copolymer,polystyrene and similar resins.

Added to the first mixture are from about 1 to about 50 parts by weightof a silane-modified thermoplastic resin of the same polymer type as atleast one of the two substantially incompatible thermoplastic resins,together with from about 0.001 to about 2.5 parts by weight of across-linking catalyst effective to cross-link the silane-modifiedthermoplastic resin, and from about 1 to about 20 parts by weight of afoaming agent to yield a blended resin composition.

The blended resin composition is thereafter extruded to form an object.The object is then exposed to a cross-linking source to form across-linked thermoplastic object wherein the cross-linked resin sheetincludes only cross-linked silane-modified thermoplastic resin.

This is followed by exposing the cross-linked object to a foaming heatsource to form a plastic foam material.

According to another feature of the present invention, there is provideda method for preparing a plastic foam material comprising thermoplasticresins, which includes preparing a first mixture containing 100 parts byweight of at least two thermoplastic resins selected from the groupconsisting of polyethylene, polypropylene, and ethylene-propylenecopolymer.

Added to the first mixture are from about 1 to about 50 parts by weightof a silane-modified thermoplastic resin of the same polymer type as atleast one of the two thermoplastic resins, together with from about0.001 to about 2.5 parts by weight of a cross-linking catalyst effectiveto cross-link the silane-modified thermoplastic resin, and from about 1to about 20 parts by weight of a foaming agent to form a blended resincomposition.

The blended resin composition is then extruded to form a thermoplasticobject. The thermoplastic object is then exposed to a cross-linkingsource to form a cross-linked thermoplastic object wherein thecross-linked thermoplastic object includes only cross-linkedsilane-modified thermoplastic resin.

This is followed by exposing the cross-linked thermoplastic object to afoaming heat source to form a plastic foam material.

According to another feature of the present invention, there is providedmethod for preparing a plastic foam material comprising thermoplasticresins, which includes forming a plurality of foamable tubes, whereineach of the plurality of foamable tubes include an inner layer and anouter layer containing thermoplastic resins.

This is followed by arranging the plurality of foamable tubesessentially parallel, supporting the tubes between dimension determiningstructures, wherein the dimension determining structures includesthickness regulating bodies effective to limit foaming in an axialdirection of the foamable tubes, exposing the foamable tubes to across-linking source to form cross-linked foamable tubes and exposingthe cross-linked foamable tubes to a foaming heat source. The foamingheat source is effective to fuse contacting parallel outer layers of theplurality of foamable tubes and also effective to initiate a foamingreaction to form the plastic foam material.

According to another feature of the present invention, there is provideda method for forming a foamable tube which includes preparing a firstmixture and preparing a second mixture. The first mixture is extrudedcoaxially together with the second mixture to form a foamable tube withthe first mixture including an inner layer and the second mixtureincluding an outer layer of the foamable tube.

According to another feature of the present invention, there is provideda method for forming a foamable tube which includes preparing a firstmixture and preparing a second mixture. The first mixture is extruded toform an inner core, while the second mixture is extruded coaxially ontothe inner core to form a foamable tube. The thus formed foamable tubeinclude an inner layer formed from the first mixture and an outer layerformed from the second mixture.

According to another feature of the present invention, there is provideda method for forming a foamable tube which includes preparing a firstmixture and a second mixture. The first mixture is extruded to form aninner core. The second mixture is dissolved in a solvent effective toform a second mixture solution. Thereafter, the inner core is coatedwith the thus formed second mixture solution. The solvent is thenremoved to provide a foamable tube wherein the inner layer includes thefirst mixture while the outer layer includes the second mixture.

According to another feature of the present invention, there is provideda method for forming foamable tubes which includes preparing a firstmixture and a second mixture. The first mixture is blended and extrudedto form a blended first object. Thereafter, the blended first object isextruded to form a tubular object for forming an inner layer.

The second resin composition mixture is extruded to form a blendedsecond object for forming an outer layer. The outer layer is thenextruded co-axially onto the tubular object to form foamable tubes.

The foamable tubes are next exposed to a cross-linking source to formcross-linked foamable tubes. The cross-linked foamable tubes arethereafter exposed to a foaming source to form foamable tubes.

According to another feature of the present invention, there is provideda method for forming foamable tubes which includes forming a firstmixture and a second mixture. The first and second mixtures areco-extruded to form foamable tubes having an inner layer and an outerlayer, wherein the inner layer includes the first mixture and the outerlayer includes the second mixture. Thereafter, the foamable tubes areexposed to a cross-linking source to form cross-linked foamable tubes.

Then the cross-linked foamable tubes are exposed to a foaming source toform said foamable tubes.

According to another feature of the present invention, there is provideda method for forming a tubular-celled resin sheet which includes formingan uncross-linked resin tube containing a foaming agent, followed bycross-linking the resin tube without activating the foaming agent.Thereafter, the resin tube is cut into substantially uniform lengths.

The uniform lengths are arranged upon a surface with axes thereofparallel to each other, with the peripheral surfaces of substantiallyall of the uniform lengths being in contact with peripheral surfaces ofadjacent uniform lengths, followed by limiting an upper end of theuniform lengths, and activating the foaming agent, whereby a unitarytubular-celled resin sheet is formed by adherence of peripheral surfacesto each other.

According to another feature of the present invention, there is provideda tubular-celled thermoplastic resin sheet which includes a plurality oflengths of a resin tube in an array in which axes thereof are parallelto each other, the plurality of lengths having been foamed to urgeperipheral surfaces of the plurality of lengths into an adhering unitarybody, and the axes being at least partly open, whereby a plurality ofopenings pass through the tubular-celled thermoplastic resin sheet.

According to another feature of the present invention, there is provideda method for preparing a plastic foam material comprising thermoplasticresins, which includes the steps of forming a plurality of foamablepieces, wherein each of the plurality of foamable pieces include a corematerial and a cover material comprising thermoplastic resins. The corematerial includes a foaming agent.

Thereafter the plurality of foamable pieces are arranged essentiallyparallel. The parallel arranged pieces are then supported betweendimension determining structures. Such dimension determining structuresinclude thickness regulating bodies effective to limit foaming in anaxial direction of the foamable pieces.

Next, the arranged, constrained pieces are exposed to a cross-linkingsource to form cross-linked foamable pieces. Afterwards, thecross-linked foamable pieces are subjected to a foaming heat sourcewhich is sufficient to fuse contacting parallel outer layers of thefoamable pieces and effective to initiate a foaming reaction to form athermoplastic foam material.

According to another feature of the present invention, there is provideda method for preparing foamable pieces comprising thermoplastic resins,which includes the steps of forming a first and a second mixture. Thefirst mixture is extruded to form a blended first object. The firstobject is then extruded to form a tubular object for forming a corematerial. The second mixture is extruded to form a blended second objectfor forming a cover material. The cover material is then extrudedco-axially onto the tubular object to form foamable pieces.

The foamable pieces are next exposed to a cross-linking source to formcross-linked foamable pieces. The cross-linked foamable pieces arethereafter exposed to a foaming source to form foamable pieces.

According to another feature of the present invention, there is provideda method for forming foamable tubes which includes forming a firstmixture and a second mixture. The first and second mixtures areco-extruded to form foamable pieces having a core material and a covermaterial, wherein the core material includes the first mixture and thecover material layer includes the second mixture.

According to another feature of the present invention, there is provideda plastic foam material comprising a cover material which includes atleast one of a thermoplastic resin and a thermoplastic resin togetherwith a foaming agent, wherein the cover material includes honeycombstructures, and a core material being integrally placed within the covermaterial.

The above, and other objects, feature and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a two-layer foam tube according to anembodiment of the present invention.

FIG. 2 is a fragmentary side view of an embodiment of a device used toproduce plastic foam material according to an embodiment of the presentinvention.

FIG. 3 is a perspective view of a rotating roller in the embodiment ofFIG. 2.

FIG. 4 is a detail plan view of a plastic foam material according to anembodiment of the present invention.

FIG. 5 is a fragmentary side view of an embodiment of a device used toproduce foam bodies according to an embodiment of the present invention.

FIG. 6 is a perspective view of a rotating roller in the embodiment ofFIG. 5.

FIG. 7 is a fragmentary side view of a feed plate according to theembodiment of FIG. 5.

FIG. 8 is a fragmentary view of a feed plate according to an embodimentof the present invention.

FIG. 9 is a detail plan view of a plastic foam material producedaccording to an embodiment of the present invention.

FIG. 10 is a detailed plan view of a plurality of tubes producedaccording to an embodiment of the present invention and arranged in astaggered formation.

FIG. 11 is a detail plan view of a plastic foam material producedaccording to an embodiment of the present invention.

FIG. 12 is a fragmentary perspective view of a sewage pipe producedusing porous foam plates produced according to an embodiment of thepresent invention.

FIG. 13 is a fragmentary side view of an embodiment of a device used toproduce foam bodies according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The plastic foam material of the present invention is composed ofthermoplastic resins and a silane-modified thermoplastic resin. Thestarting material for forming the plastic foam material may furtherinclude specified amounts of cross linking catalysts andthermodecomposition foaming agents.

EMBODIMENT 1

According to a first embodiment, a plastic foam material includes ablended resin composition containing thermoplastic resins and asilane-modified thermoplastic resin. The thermoplastic resins include atleast two substantially incompatible, uncross-linked thermoplasticresins (hereinafter referred to as "two incompatible thermoplasticresins").

Essentially, the blended resin composition consists of 100 parts byweight of at least two substantially incompatible thermoplastic resins,from about 1 to about 50 parts by weight of a silane-modified resin;from about 0.001 to about 2.5 parts by weight of a cross-linkingcatalyst for use in a silane compound and from about 1 to about 20 partsby weight of a thermodecomposition foaming agent (hereinafter referredto as "foaming agent").

The plastic foam material exhibiting superior heat-tolerance andcompression strength properties of the present invention is composed ofthe following elements, described hereinafter. All percentageshereinafter referred to are in weight terms (parts by weight) unlessother, vise stated.

Thermoplastic resins

The thermoplastic resins for use in the blended resin composition areuncross-linked and need not be limited to specific resins, so long asthe resins are highly foamable.

The two substantially incompatible thermoplastic resins for use in theblended resin composition include at least two members selected from thegroup consisting of a polyethylene, polypropylene, a ethylene-propylenecopolymer, an ethylene-vinyl acetate copolymer, polystyrene, and similarcompounds.

The term "propylene" includes at least one of a homopolypropylene, blockpolypropylene, and random polypropylene. A "block polypropylene includesan ethylene-propylene copolymer containing small amounts of an ethylenecomponent. Random polypropylene includes a static polypropylenecopolymer containing small amounts of an ethylene component. It ispreferred that the two substantially incompatible thermoplastic resinsinclude at least two members selected from the group consisting ofpolyethylene, polypropylene, and polystyrene. In general, highlyfoamable thermoplastic resins provide for increased "foaming stability".

The degree of polymerization of the aforementioned polyethylene shouldbe from about 500 to 6000, more preferably from 600 to 5000. The degreeof polymerization of the aforementioned polypropylene should be fromabout 800 to 12000, more preferably from 1000 to 10000. The degree ofpolymerization of the aforementioned polystyrene should be from about250 to 5000, more preferably from 1000 to 4000. Outside these preferredranges, the dispersion of the silane-modified, cross-linked, plasticresin is substantially compromised.

Melt index

The melt index (hereinafter referred to as "MI" as g per 10 minutes) ofthe two substantially incompatible thermoplastic resins is determined inaccordance with Japanese Industrial Standard (JIS) K7210.

The MI of the two substantially incompatible thermoplastic resinspreferably ranges from about 0.1 to about 50 g per 10 minutes, and morepreferably, from about 1 to about 15 g/10 minutes. When the MI of thetwo substantially incompatible thermoplastic resin exceeds 50 or fallsbelow 0.1 g/10 minutes, dispersion of the silane-modified thermoplasticresin substantially deteriorates, which leads to poor foamingperformance.

If polyethylene and polypropylene are selected as the two substantiallyincompatible thermoplastic resins for use in the blended resincomposition, then a density of each respective resin is preferablyhigher than about 0.91 and 0.89 g/cm³, respectively. The plastic foammaterial becomes markedly weak and exhibits inferior heat resistanceproperties when the density of at least one of the polyethylene andpolypropylene resins falls below the cited values.

Similarly, if ethylene-vinyl acetate copolymer and polystyrene areselected to form the two substantially incompatible thermoplastic resinsfor use in the blended resin composition, then a density of eachrespective resin is preferably from about 0.92 to about 0.95 g/cm³ andfrom about 1.040 to about 1.60 g/cm³, respectively.

When the density of the ethylene-vinyl acetate copolymer falls below0.92 g/cm³, the rigidity of the plastic foam material deteriorates. Asimilar effect is observed when the density of the polystyrene fallsbelow 1.040 g/cm³.

On the other hand, when the density of the ethylene-vinyl acetatecopolymer exceeds 0.95 g/cm³, the crystallinity of this polymer becomehigh while its dispersion substantially deteriorates. Similar to theethylene-vinyl acetate copolymer, the crystallinity of polystyrenebecomes high with a significant decrease in its dispersion, when thedensity of the polystyrene exceeds 1.60 g/cm³.

The two thermoplastic resins for use in the present invention accordingto the first embodiment, are characterized as being substantiallyincompatible with each other. The two substantially incompatiblethermoplastic resins have individual melt indexes (MI). The differencein the MI between the two substantially incompatible thermoplasticresins (hereinafter referred to as "MI'") varies from about 3 to about15 g per 10 minutes.

The weight ratio of the two substantially incompatible thermoplasticresins should be from about 2:8 to about 8:2.

When the MI' exceeds 15 g/10 minutes, the "sea and island" structure ofthe thermoplastic resin composition becomes "rough". Essentially, thesize of the "island" becomes too large with poor foaming performance asa consequence.

On the other hand, when the difference in the melt index between the twosubstantially incompatible thermoplastic resins becomes small, thedispersed "sea and island" structure becomes too small or non-existentand poor foaming performance is again the consequence.

When the MI' is from about 3 to about 15 g per 10 minutes, a uniformsea-and-island structure with fine particle size is observed. It ispreferable that MI' be from about 7 to about 11 g per 10 minutes.

For purposes of this invention, the term "solubility parameter" refersto the value calculated from the following formula:

    σ=ρΣFi/M.

where,

σ is the solubility parameter,

ρ is the density of the thermoplastic resins,

M is the molecular weight of the monomer which forms the thermoplasticresins, and

Fi is the molar attraction constant of the constituent groups of themonomer.

The degree of incompatibility between the two substantially incompatiblethermoplastic resins can be measured and represented as a "solubilityparameter". When the degree of incompatibility, i.e., solubilityparameter between the aforementioned two substantially incompatibleresins falls below 0.1, there is an absence of a uniform sea-structure.

On the other hand, when the degree of incompatibility between theaforementioned two substantially incompatible thermoplastic resinsexceeds 2, the difference in the solubility parameter between the tworesins becomes small. In this case, no sea-and-island structure isformed. The difference in the solubility parameter between the twosubstantially incompatible thermoplastic resins should preferably befrom about 0.1 to 2, and more preferably, from about 0.2 to 1.5.

It is preferred that the weight ratio of the two substantiallyincompatible resins be limited to be from about 2:8 to about 8:2, andthe areas of the island and the sea in the "sea-and-island" structure beapproximately the same.

When a weight fraction of one of the two substantially incompatiblethermoplastic resins is high, it becomes exceedingly difficult to obtaina suitable shear viscosity for forming foam. In order to uniformlydisperse one of the substantially incompatible thermoplastic resins intothe other substantially incompatible thermoplastic resin, it ispreferred that the weight ratio of the two substantially incompatibleresins be from about 4:6 to about 6:4, and more preferably, 5:5.

Silane-modified thermoplastic resins

As mentioned previously, the plastic foam material of the presentinvention is composed of a blended resin composition. The blended resincomposition, in turn, contains at least two thermoplastic resins (i.e.,substantially incompatible or compatible) and a silane-modifiedthermoplastic resin.

The silane-modified thermoplastic resin includes a resin type similar toat least one of the two substantially incompatible thermoplastic resins.The silane-modified thermoplastic resin is capable of being cross-linkedwith or without the aid of a cross-linking catalyst. The gel fraction ofthe silane-modified thermoplastic resin, after cross-linking, should befrom about 60 to about 85 parts by weight.

The difference in the MI between the silane-modified thermoplastic resinand the substantially incompatible resin of the same polymer type(hereinafter referred to as MI") is less than 1 g per 10 minutes.

The above mentioned silane-modified resin may include at least one of aconventional silane-modified thermoplastic resin exemplified by asilane-modified thermoplastic resin of polyethylene, a silane-modifiedthermoplastic resin of polypropylene, a silane-modified thermoplasticresin of an ethylene-vinyl acetate copolymer, a silane-modifiedthermoplastic resin of polystyrene.

Because of their superior foam forming properties, at least one ofpolyethylene, polypropylene, and polystyrene as the silane-modifiedthermoplastic resin is preferred.

For purposes of this embodiment, the term "silane-modified thermoplasticresin" includes thermoplastic resins modified by an unsaturated silanecompound by graft polymerization.

An unsaturated silane compound is a compound given by the followinggeneral formula:

    R'SiR".sub.m Y.sub.3-m.

where,

R' includes an organic functional group, e.g., an alkenyl group such asa vinyl group, an allyl group, a propenyl group or a cyclohexanyl group;a glycidyl group; an amino group; a methacrylic group; a halogenatedalkyl group such as a γ-chloroethyl group or γ-bromoethyl group;

R" includes one of an alkyl group (a saturated aliphatic hydrocarbongroup), an aryl group (an aromatic hydrocarbon group), such as a methyl,ethyl, propyl, butyl, phenyl groups, and similar groups;

m represents one of 0, 1, or 2;

Y is an organic group which can be hydrolyzed;

For example, Y may be one of a methoxy group, ethoxy group, formyloxygroup, acetoxy group, propionoxy group, alkyl group, aryl amino group,etc. When m equals 0 or 1, Y can be either identical or different.

It is preferred that the aforementioned unsaturated silane compound berepresented by a general formula

    CH.sub.2 ═CHSi(OA).sub.3

where,

A includes a hydrocarbon group with 1 to 8 carbon atoms, and morepreferably, from 1 to 4 carbon atoms. For example, CH₂ ═CHSi(OA)₃ may bevinyl trimethoxysilane, vinyl triethoxysilane, vinyl triacetoxysilane,etc.

In cases where the silane-modified thermoplastic resin contains amethoxy group, the methoxy group hydrolyzes to yield a hydroxyl groupupon contacting water.

The, thus obtained hydroxyl group, in turn, can react with a hydroxylgroup of another molecule to form a Si-O-Si bond. In essence, twosilane-modified thermoplastic resins become cross-linked. It ispreferred that a cross-linking catalyst be used to promote thecross-linking reactions.

The gel fraction of the silane-modified thermoplastic resin aftercross-linking is limited to from 60 to 85 parts by weight, and morepreferably, from 70 to 80 parts by weight. When the gel fraction of thesilane-modified thermoplastic resin, after cross-linking, is below 60parts by weight, the cross-linking density is low, and the foamingproperties of the blended resin composition are adversely effected.

To substantially improve the foaming stability of the cross-linkedresin, it is advisable to limit the gel fraction of the silane-modifiedthermoplastic resin to from about 70 to about 80 parts by weight.

The gel fraction is used to indicate the degree of cross-linking and isexpressed as the parts by weightage of the residue obtained after xyleneextraction at 120° C. To measure the residue resulting from the xyleneextraction, a predetermined amount of plastic foam material was immersedin predetermined amount of xylene and kept there at 120° C. for 24hours. This effectively dissolves the uncross-linked portion of theplastic foam material. The contents of the container were then pouredthrough a mesh screen to effectively separate the residue from solution.The resulting residue retained by the screen was collected, dried at 80°C. and 10 mm Hg pressure for 5 hours, and weighed.

The amount (parts by weightage) of the resulting residue is calculatedby utilizing the formula:

    Residue (parts by weight)=(b/a)×100

where,

a equals initial weight of the sample plastic foam material (gram)

b equals weight of the dried residue collected (gram).

According to the present invention, MI" should be less than 1 g per 10minutes.

When MI" exceeds 1 g per 10 minutes, it becomes increasingly difficultto disperse the silane-modified thermoplastic resin into theaforementioned thermoplastic resin of the same polymer type.

When the amount of the silane-modified thermoplastic resin added to theblended resin composition exceeds 50 parts by weight, the dimensionalstability of the plastic foam material substantially deteriorates.

On the other hand, when the total content of the silane-modifiedthermoplastic resin added falls below 1 part by weight to 100 parts byweight of the two thermoplastic resins, the blended resin compositionexhibits decreased elongation viscosity which is required for formingfoam.

In order to effectively generate sufficient foam, the blended resincomposition should have sufficient elongation viscosity. When theelongation viscosity is decreased, as when the amount of thesilane-modified thermoplastic resin added to 100 parts by weight of thetwo thermoplastic resins falls below 1 part by weight, the attendantdecrease in elongation viscosity leads to a halt in foam production.

It is preferable that the amount of the silane-modified thermoplasticresin added to 100 parts by weight of the two thermoplastic resins befrom about 5 to about 40 parts by weight, and more preferably from about10 to 30 part by weight.

Cross-linking catalyst

The blended resin composition may further include a cross-linkingcatalyst for use in a silane compound.

The cross-linking catalyst for use in the present invention can includea cross-linking catalyst effective to catalyze a cross-linking reactionbetween the silane-modified thermoplastic resin molecules. It need notbe limited to any specific cross-linking catalyst. It includes at leastone of a dibutyl tin diacetate, dibutyl tin dilaurate, dioctyl tindilaurate, tin octanoate, tin oleate, lead octanoate, 2-ethyl hexanezinc, cobalt octanoate, lead naphtenate, zinc caprylate, zinc stearate,etc.

The foaming property (the ability of the blended resin composition toform foam at predetermined temperatures) of the blended resincomposition deteriorates when the amount of the cross-linking catalystper 100 parts by weight of the blended resin composition exceeds 2.5parts by weight. The degree of cross-linking between the constituents toform the plastic foam material is then insufficient.

Similarly, if the amount of the cross-linking catalyst per 100 parts byweight of the blended resin composition falls below 0.001 parts byweight, the cross-linking reaction rate between silane-modifiedthermoplastic resin molecules is severely hampered and slowed.

Accordingly, it is preferable that the amount of the cross-linkingcatalyst added to 100 parts by weight of the total of the twothermoplastic resins and silane-modified thermoplastic resin be fromabout 0.001 to about 2.5 parts by weight, and more preferably from about0.1 to 1.5 parts by weight.

Foaming Agent

A thermodecomposition foaming agent capable of decomposing at hightemperatures is added to the blended resin composition. The foamingagent may include one of a conventional thermodecomposition foamingagent exemplified by an azodicarbonamide (1,1'-azobisformamide),azobisisobutylonitrile, N,N'-dinitrosopentamethylene tetramine,4,4-oxybis(benzene sulfonyl hydrazide) barium azodicarboxylate,trihydrazinotriazine, benzene sulfonyl hydrazide, toluene sulfonylhydrazide, and similar compounds.

Azodicarbonamide (1,1'-azobisformamide) is preferred because of itssensitive decomposition peak temperature.

The ability of the blended resin composition to form foam upon thermalinteraction is greatly compromised when the amount of the foaming agentin the blended resin composition falls below 1 part by weight.

On the other hand, the overall strength of the resulting plastic foammaterial deteriorates when the amount of the foaming agent in 100 partsby weight of the two thermoplastic resins and the silane-modifiedthermoplastic resin exceeds 20 parts by weight. Additionally, theability to form uniform foam cells is severely compromised when theamount of the foaming agent exceeds 20 parts by weight. It is preferablethat the total amount of the foaming agent be from about 5 to about 15parts by weight.

Other Additives

In addition to the above noted compounds, the blended resin compositionmay further contain glass fiber. The glass fiber is added to improve theoverall strength and the dimensional stability of plastic foam materialand its derived products.

The amount of the glass fiber added to the blended resin compositionshould be such that its addition does not harm the physical propertiesof the plastic foam material composed of a blended resin composition.

When a diameter of the glass fiber exceeds 30 μm, the glass fiber makeskneading the blended resin composition exceedingly difficult. On theother hand, when the diameter of the glass fiber falls below about 5 μm,the glass fiber breaks easily. This, in turn, substantially weakens theplastic foam material.

Accordingly, it is recommended that the average diameter of the addedglass fiber be from about 5 to about 30 μm. It is preferable that thediameter of the glass fiber be from about 7 to about 20 μm.

When the length of the glass fiber is too great, the cell walls of theplastic foam material may be punctured by the glass fiber, causing asubstantial decrease in the overall volume of foam produced.

On the other hand, when the length of the glass fiber is too short, theplastic foam material fails to exhibit a suitable improvement in overallstrength. Accordingly, it is preferred that the length of the glassfiber be from about 0.1 to about 10 mm, and more preferably, from about0.5 to 5 mm.

When the amount of the added glass fiber exceeds 20 parts by weight ofthe two olefin based resins and the silane-modified thermoplastic resin,the ability of the blended resin composition to form foam issubstantially impaired. Alternatively, when the amount of the addedglass fiber falls below 1 part by weight, the overall strength of theplastic foam material is impaired. Thus, the amount of the glass fiberto be added is preferably from about 1 to about 20 parts by weight, to100 parts by weight of the two thermoplastic resins and thesilane-modified thermoplastic resin.

EMBODIMENT 2

According to a second embodiment of the present invention, the plasticfoam material is composed of a blended resin composition which containsthermoplastic resins and a silane-modified thermoplastic resin. Theblended resin composition consists of 100 parts by weight of at leasttwo thermoplastic resins, from about 1 to about 50 parts by weight of asilane-modified thermoplastic resin and a thermodecomposition foamingagent.

Described hereinafter, are the various components which form the plasticfoam material in accordance with the second embodiment. All parts byweightages hereinafter referred to are in weight terms unless otherwisestated.

Thermoplastic resins

Unlike the first embodiment, the uncross-linked thermoplastic resins foruse in this embodiment need not be substantially incompatible. Similarto the first embodiment, the thermoplastic resins include at least tworesins selected from a group exemplified by polyethylene, polypropylene,and an ethylene-propylene copolymer.

The degree of polymerization of the polyethylene should be from 1,000 toabout 10,000, and more preferably, from about 2,000 to 5,000. Thedispersion of the polyethylene deteriorates when the degree ofpolymerization of the polyethylene is either below 1,000 or above10,000. The deterioration in the dispersion of the polyethylene, inturn, impairs the dispersion of the silane-modified thermoplasticresins.

The degree of polymerization of the polypropylene is from about 5,000 toabout 12,000, and preferably, from about 7,000 to 10,000.

The degree of polymerization of the ethylene-propylene copolymerpolyethylene should be from 8,000 to about 10,000.

Melt index (MI)

The MI of the two thermoplastic resins is determined in accordance withJapanese Industrial Standard (JIS) K7210.

The MI of the polyethylene is from about 0.6 to about 20 g per 10minutes, and preferably, from about 5 to 15 g per 10 minutes. Thedispersion of the polyethylene deteriorates when the MI of thepolyethylene falls below 5 g per 10 minutes. The deterioration in thedispersion of the polyethylene, in turn, impairs dispersion of thesilane-modified thermoplastic resin.

The MI of the polypropylene should be from about 2 to about 25 g per 10minutes. Outside the aforementioned range, the dispersion of thepolypropylene and the silane-modified thermoplastic resins is severelyhampered.

The MI of the ethylene-propylene copolymer should be from about 3 to 8 gper 10 minutes. Outside these preferred ranges, the dispersions of thesilane-modified thermoplastic resin and the ethylene-propylene copolymerare severely compromised.

The density of the polyethylene should be higher than 0.94 g/cm³. Theplastic foam material becomes markedly weak and exhibits inferiorfoaming performance when the density of the polyethylene falls below0.94 g/cm³.

The density of the polypropylene should be higher than 0.90 g/cm³.

Similar to the polyethylene and the polypropylene, the plastic foammaterial becomes markedly weak and exhibits inferior foaming performancewhen the density of the ethylene-propylene copolymer falls below 0.90g/m³.

It is preferred that the ethylene content in the thermoplastic resins isfrom about 20 to about 80 parts by weight. The dispersion of thesilane-modified thermoplastic is impaired when the ethylene content inthe two thermoplastic resins for use in the blended resin composition,falls outside the aforementioned preferred range.

Silane-modified thermoplastic resin

The silane-modified thermoplastic resin according to this embodimentmust be compatible with at least one of polyethylene, polypropylene, andan ethylene-propylene copolymer.

The silane-modified thermoplastic resin may be one of a silane-modifiedpolyethylene, a silane-modified polypropylene, a silane-modified,ethylene-propylene copolymer, etc.

The silane-modified thermoplastic resin may be prepared using aconventional method. For example, a silane-modified polyethylene can beprepared by the reaction of polyethylene with an unsaturated silanecompound and an organic peroxide. The unsaturated silane compound may berepresented by

    RSiR'Y.sub.2

where,

R includes an organic functional group, e.g., an alkenyl group such as avinyl group, an allyl group, a propenyl group or a cyclohexanyl group; aglycidyl group; an amino group; a methacrylic group; a halogenated alkylgroup such as a γ-chloroethyl group or γ-bromoethyl group;

Y represents an organic group that can be hydrolyzed, and

R' represents either a R group or a Y group as defined above.

When the amount of the silane-modified thermoplastic resin falls below 1part by weight per 100 parts by weight of the two thermoplastic resins,an elongation viscosity of the blended resin composition duringthermofoaming becomes insufficient. The decrease in elongation viscositysubstantially reduces the output of the plastic foam material.

On the other hand, when the amount of the silane-modified thermoplasticresin exceeds 50 parts by weight per 100 parts by weight of thethermoplastic resin, there is a concurrent enhancement in the foamingstability. Further, the ground material cannot be re-extruded and thusit becomes difficult to reuse and reutilize the plastic foam material.

The amount of the silane-modified thermoplastic resin according to thisembodiment should preferably be from about 1 to about 50, and morepreferably, from about 5 to about 30 parts by weight per 100 parts byweight of the total polyolefin based thermoplastics.

Cross-linking catalyst

The cross-linking catalysts are similar to those described previously.

If the amount of the cross-linking catalyst added is insufficient, thecross-linking reaction between the silane-modified thermoplastic resinswill not progress. On the other hand, if the amount of the cross-linkingcatalyst added is too large, the ability of the blended resincomposition to produce foam upon thermal decomposition deteriorates.

Accordingly, the amount of the cross-linking catalyst to be added to theblended resin composition should preferably be from about 0.001 to about10, and more preferably, from about 0.01 to 5 parts by weight of thetotal amount of the two polyolefin based thermoplastics and thesilane-modified thermoplastic resin.

Foaming Agent

Similar to the first embodiment, the blended resin composition mayfurther include specified amounts of a thermodecomposition foamingagent.

The foaming agent must be capable of decomposing at sufficiently hightemperatures to yield a plastic foam material. Similar to the otherembodiments, the foaming agent includes at least one of aazodicarbonamide (1,1'-azobisformamide), azobisisobutylonitrile,N,N'-dinitrosopentamethylene tetramine, 4,4'-oxybis(benzene sulfonylhydrazide) also known as p,p'-oxybis(benzene sulfonyl hydrazide), bariumazodicarboxylate, trihydrazinotriazine, benzene sulfonyl hydrazide,toluene sulfonyl hydrazide.

The ability of the blended resin composition to form foam upon thermaldecomposition is greatly compromised when the amount of the foamingagent in the blended resin composition falls below 1 part by weight.

On the other hand, the overall strength of the resultant plastic foammaterial deteriorates when the amount of the foaming agent in theblended resin composition exceeds 20 parts by weight. Additionally, theability to form uniform foam cells is severely compromised when theamount of the foaming agent in the blended resin composition exceeds 20parts by weight. Accordingly, the total amount of the foaming agent per100 parts by weight of the two thermoplastic resins and thesilane-modified thermoplastic resin should be from about 1 to about 20parts by weight.

Other additives

In addition to the aforementioned polyolefin based thermoplastic resins,the silane-modified thermoplastic resin, the cross-linking catalyst andthe foaming agent, the blended resin composition may further includespecified amounts of glass fiber.

Similar to the first embodiment, the amount of the glass fiber added tothe blended resin composition should be such that its addition does notharm the physical properties of the plastic foam material composed of ablended resin composition.

The glass fiber is added to improve the overall strength and dimensionalstability of the plastic foam material.

The preferred diameters of the glass fiber are similar to the firstembodiment. The amount of the glass fiber added to 100 parts by weightof the two thermoplastic resins and the silane-modified thermoplasticresin is similar to the first embodiment.

Similar to the previous embodiment, when the length of the glass fiberis too great, the cell walls of the plastic foam material can bepunctured by the glass fiber, causing a substantial decrease in theoverall volume of foam produced.

On the other hand, when the length of the glass fiber is short, theplastic foam material fails to exhibit the desired improvement inoverall strength. Accordingly, it is preferred that the length of theglass fiber be at least 3 mm.

Preparation of the blended resin composition and its derived plasticfoam material:

For the purposes of this invention, the blended resin composition isprepared from one of two possible general types of compositions.

In the first (Type I, in accordance with embodiment 2), predeterminedamounts of (1) two uncross-linked thermoplastic resins selected from thegroup consisting of polyethylene, polypropylene, and anethylene-propylene copolymer, (2) a silane-modified thermoplastic resin,(3) a cross-linking catalyst for the silane-modified thermoplasticresin, and (4) a thermodecomposing foaming agent are mixed by kneadingin a conventional kneading machine.

In the second (Type II, similar to embodiment 1), predetermined amountsof (1) two substantially incompatible uncross-linked thermoplasticresins, (2) a silane-modified thermoplastic resin, (3) a cross-linkingcatalyst for the silane-modified thermoplastic resin and (4) athermodecomposing foaming agent are mixed by means of melt kneading in aconventional kneading machine.

The thermoplastic resin of either type composition is kneaded and moldedinto shapes of plate, sheet, or tube, etc. It is usually molded into athermoplastic resin sheet at a temperature sufficient to preventdecomposition of the foaming agent or to assure that there is nosubstantial progression towards the cross-linking. The method used toform the articles may include extrusion and similar processes.

A kneading machine for use in the mixing may include a conventionalsingle screw extruder, a twin screw extruder, a Banbury type mixer, akneader mixer, a roller, or any other suitable apparatus.

The kneaded resin of either type is rolled or extruded to form a resinsheet. The resulting thermoplastic resin sheet is next subjected to aprocess called "water treatment" in order to effectively cross-link onlythe silane-modified, cross-linked resin component of the thermoplasticresin sheet. During the water treatment, the thermoplastic resin sheetis heated to a temperature sufficient to initiate the cross-linkingcatalyst but lower than the thermodecomposition temperature of thefoaming agent. Only the molecules of the silane-modified thermoplasticresin are cross-linked together. After the cross-linking reaction, theblended resin composition is heated to a temperature higher than thedecomposition temperature of the type foaming agent.

During the water treatment, the foamable resin composition can betreated by methods other than immersing the blended resin composition inwater. For example, the blended resin composition can be treated byexposure to steam.

The temperature of the water treatment for Type I compositions shouldgenerally be from about 50° C. to about 130° C. Pressurized conditionsare necessary for temperatures above 100° C. If the temperature of thewater is too low, the reaction rate is too slow and the time needed tocomplete cross-linking is too long. The preferred time for watertreatment is at least 2 hours to assure complete cross-linking of thesilane-modified thermoplastic resin component.

The temperature of the water treatment for Type II blended resincompositions is preferably from about 50° to about 130° C., and morepreferably, from about 80° to 120° C. When the water treatmenttemperature is too high, the resin composition tends to fuse together.This leads to poor expansion ratios during the foaming process. If thetemperature of the water treatment is too low, the cross-linkingreaction time is too long and the cross-linking reaction may not go todesired completion.

The thus obtained thermoplastic resin sheet of either type compositionwith the cross-linked silane-modified thermoplastic resin component isthen heated in an oven above the decomposition temperature of thefoaming agent. This heating step forms a foam material upon thermaldecomposition of the foaming agent. Alternatively, the resin sheet withthe cross-linked silane-modified thermoplastic resin component may beplaced in a heating roller and heated to form the plastic foam material.Alternatively, the resin sheet with the cross-linked silane-modifiedthermoplastic resin component can be placed in a mold and heated to formthe foam material.

According to this invention, for Type II compositions, of the twouncross-linked thermoplastic resins, the particle size of the resin withthe lower melt index importantly affects the melt-kneading of theblended resin. When the particle size of the uncross-linked polyolefinbased resin with the lower melt-index is too large, the dispersion ofthe uncross-linked thermoplastic resins becomes poor and the foamingproperty of the blended resin composition deteriorates.

It is preferred that the particle size of the uncross-linkedthermoplastic resin with lower melt-index be smaller than about 50 μm,desirably, smaller than about 10 μm.

The heating time to decompose the foaming agent should be sufficientlylong to assure complete foaming/foam formation/foam generation. It ispreferred that the heating time be longer than 30 seconds.

After the water treatment of the blended resin of Type I composition,but before the thermal foaming step, the b/ended resin composition maybe covered by an inorganic fabric sheet. This is desirable because itforms a plastic foam material with less thermal deformation.

After the water treatment of the polyolefin resin of Type IIcomposition, but before the thermal foaming step, the foamable resincomposition may be sandwiched between two inorganic fabric sheets. Thisis desirable because it improves the dimensional stability of theresulting plastic foam material.

The aforementioned inorganic fabric sheets can be any conventionalinorganic fabric sheet. For example, a sheet made from rock wool, asheet made from carbon fiber, a glass cloth, a surfacing sheet may besuitably used as an inorganic fabric sheet noted above. The glass clothis woven of glass thread which is obtained by converging glass fibers.The surfacing sheet is obtained from piling glass fibers randomly to auniform thickness and binding with an adhesive.

Examples embodying the blended resin composition of the presentinvention are described hereinafter.

EXAMPLES 1-6 AND 9-10 AND COMPARATIVE EXAMPLES 1-2

Predetermined amounts of high density polyethylene, polypropylene,ethylene-propylenecopolymer, silane-modifiedpolyethylene,silane-modified polypropylene, azodicarbonamide, and dibutyl tindilaurate as set forth in Table 1 are mixed in a twin-screw extruder togive various resin compositions of Type I.

The screw of the twin-screw extruder used has a diameter of 30 mm. Theblended resin composition obtained from the extruder is cylindrical andis 2 mm in diameter.

Subsequently, the blended resin composition obtained from the extruderis rolled by a cooling roller to form a 0.7 mm thick material. The 0.7mm thick material is cut and pelletized by a right angular pelletizer.The pellets obtained are then subjected to a water treatment for 2 hourswhere the temperature of the water for immersion is kept at 99° C.

The water-treated, pelletized, blended resin composition is put in anair oven at 210° C. where the pellets of the resin composition are fusedtogether, to allow the formation of plastic foams to take place and theplastic foamed material of the resin composition is obtained aftercooling in air.

The expansion ratio, the melt index, and the parts by weightage ofshrinkage of the plastic foam material of the resin composition aremeasured following the methods described below and the results are shownin Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                  Comparative                                     Example                       Example                                         1  2  3  4  5  6  7  8  9  10 1   2                           __________________________________________________________________________    High density polyethylene                                                                     50 50 50 50 -- 40 50 50 50 50 50  50                          Polypropylene   50 50 50 -- 50 40 50 50 50 50 50  50                          Ethylene-propylene copolymer                                                                  -- -- -- 50 50 20 -- -- -- -- --  --                          Silane-modified polyethylene                                                                  5  15 -- 15 -- 15 5  5  30 40 --  60                          Silane-modified polypropylene                                                                 -- -- 15 -- 15 -- -- -- -- -- --  --                          Dibutyl tin dilaurate                                                                         0.25                                                                             0.75                                                                             0.75                                                                             0.75                                                                             0.75                                                                             0.75                                                                             0.25                                                                             0.25                                                                             1.50                                                                             1.50                                                                             --  1.50                        Azodicarbonamide                                                                              8  8  8  8  8  8  8  8  8  8  8   8                           Expansion Ratio 10.2                                                                             11.0                                                                             9.8                                                                              10.6                                                                             9.4                                                                              10.8                                                                             10.2                                                                             10.2                                                                             10.0                                                                             9.0                                                                              NMF 7.0                         Melt viscoxity index (g/10 min.)                                                              0.8                                                                              0.3                                                                              1.0                                                                              0.2                                                                              0.9                                                                              0.7                                                                              0.3                                                                              0.3                                                                              0.08                                                                             0.05                                                                             2.8 0.02                        Percentage of shrinkage (%)                                                                   0.10                                                                             0.15                                                                             0.10                                                                             0.15                                                                             0.10                                                                             0.10                                                                             0.05                                                                             0.10                                                                             0.30                                                                             0.50                                                                             0.05                                                                              0.75                        __________________________________________________________________________     Note: NMF means "Not Meaningful." No stable foaming.                     

EXAMPLE 7

The water-treated, pelletized, blended resin composition obtained fromExample 1 was layered with glass paper FEO-025 (approximate weight, 25g/m³, provided by Oribest Co, Ltd.) to give a layered, blended resincomposition. This was further clamped by a Chuko Flow G-Type belt(manufactured by Chub Kasei Kogyo) and heated to form a glasspaper-layered, plastic foam material that was 180 mm long, 300 mm wide,and 5 mm thick. The approximate weight of the glass paper-layered,plastic foam material obtained was 700 g/m².

EXAMPLE 8

Similar to Example 7, except that a surfacing sheet SM-3600-E(approximate weight, 30 g/m², provided by Asahi Glass) was substitutedfor the glass paper. This example yielded a plastic foam material thatwas also 180 mm long, 300 mm wide, and 5 mm thick. The approximateweight of the glass paper-layered, plastic foam material obtained wasalso 700 g/m².

EXAMPLES 11-22 AND 25 AND COMPARATIVE EXAMPLES 3-10

Predetermined amounts of the uncross-linked plastic resins polyethylene,polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetatecopolymer, polystyrene and a silane-modified cross-linked plastic resinas set forth by Table 2, in addition to 0.1 part by weight of dibutyltin dilaurate, and 15 parts by weight of azodicarbonamide are fed to atumbler and mixed to from various mixtures.

                                      TABLE 2                                     __________________________________________________________________________             Melt                                                                              Example                       Comparative Example                         Index                                                                             11                                                                              12                                                                              13                                                                              14                                                                              15                                                                              16                                                                              17                                                                              18                                                                              19                                                                              20                                                                              21                                                                              22                                                                              23                                                                              24                                                                              25                                                                              3 4 5 6 7  8  9  10                __________________________________________________________________________    Polyethylene 1                                                                         1.5 50                                                                              50                                                                              30                                                                              70                50                                                                              50                                                                              50                                                                              50                                                                              10                                                                              90     50 50 50                Polyethylene 2                                                                         9                     50                                                                              50                                                                              50                                         Polyethylene 3                                                                         11                                           50                                                                            50                      Polyethylene 4                                                                         20          50                                                       Polyethylene 5                                                                         0.35            50                                                   Polyethylene 6                                                                         0.6           50                                                     Polypropylene 1                                                                        1.2                                          50                      Polypropylene 2                                                                        11  50                                                                              50                                                                              70                                                                              30                                                                              50                                                                              50                                                                              50                                                                              50                                                                              50      50                                                                              50                                                                              50                                                                              50                                                                              90       10                                                                            50                                                                            50 50 50                Polypropylene 3                                                                        2.8                   50                                             Ethylene vinyl                                                                         3                 50    50                                           acetate                                                                       copolymer                                                                     Polystyrene                                                                            3                   50    50                                         Silane modified                                                                        10                    20                                                                              20                                                                              20                 20                      cross-linked                                                                  polyethylene                                                                  Silane modified                                                                        10  20                                                                               5                                                                              20                                                                              20                                                                              20                                                                              20                                                                              20                                                                              20                                                                              20      20                                                                              20                                                                              30  20       20                                                                            20                                                                            20 60 80                cross-linked                                                                  polypropylene                                                                 __________________________________________________________________________

The azodicarbonamide is used as the thermodecomposition type foamingagent whereas the dibutyl tin dilaurate is used as the cross-linkingcatalyst. The mixtures are mixed in a twin-screw extruder andmelt-kneaded at 180° C. to give various Type II resin compositions.

Similar to the Type I composition examples above, the screw of thetwin-screw extruder used had a diameter of 30 mm. The blended resincomposition obtained from the extruder was cylindric and was 2 mm indiameter.

Subsequently, the blended resin composition obtained from the extruderwas rolled by a cooling roller to form a 0.7 mm thick material. The 0.7mm thick material was cut and pelletized by a right angular pelletizer.The pellets obtained were then subjected to a water treatment for 2hours where the temperature of the water for immersion is kept at 99° C.

The water-treated, pelletized, blended resin composition was put in anair oven at 210° C. where the pellets of the resin composition werefused together, to allow the formation of plastic foams to take placeand the plastic foamed material of the resin composition was obtainedafter cooling in air.

The expansion ratio, formability, and percentage of shrinkage of theplastic foam material of the resin composition were measured followingthe methods described below and the results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                 Expansion Thermal                                                             Ratio     Shrinkage Formability                                      ______________________________________                                        Example                                                                       11         30          0.25      100                                          12         15          0.15      120                                          13         25          0.25      80                                           14         15          0.25      100                                          15         10          0.25      140                                          16         25          0.25      80                                           17         20          0.25      70                                           18         20          0.25      100                                          19         20          0.25      100                                          20         25          0.25      110                                          21         25          0.25      100                                          22         20          0.25      100                                          23         30          0.25      60                                           24         30          0.25      60                                           25         30          0.35      50                                           Comparative                                                                   Example                                                                        3         3           0.05      130                                           4         5           0.25      50                                            5         2           0.25      50                                            6         10          0.25      120                                           7         11          0.25      120                                           8         11          0.25      40                                            9         20          0.75      20                                           10         15          0.90      10                                           ______________________________________                                    

EXAMPLE 23

The water-treated, pelletized, blended resin composition obtained fromExample 11 was layered with glass paper FEO-025 (approximate weight, 25g/m², provided by Oribest Co., Ltd) to give a layered, blended resincomposition. This was further clamped by a Chuko Flow G-Type belt(manufactured by Chuko Kasei Kogyo) and heated to form a glasspaper-layered, plastic foam material that was 180 mm long, 300 mm wide,and 5 mm thick. The approximate weight of the glass paper-layered,plastic foam material obtained was 700 g/m².

EXAMPLE 24

Similar to Example 23, except that a surfacing sheet SM-3600-E(approximate weight, 30 g/m², provided by Asahi Glass) was substitutedfor the glass paper. This example yielded a plastic foam material thatwas also 180 mm long, 300 mm wide, and 5 mm thick. The approximateweight of the glass paper-layered, plastic foam material obtained wasalso 700 g/m².

Expansion Ratio

The density of the pellet of the pelletized, blended resin compositionbefore the formation of the plastic foam material was measured anddesignated "d₁ ". After the formation of plastic foam material, adensity of the plastic foam material composed of the blended resincomposition was measured and designated "d₂ ". The expansion ratio wascalculated as the ratio expressed as d₁ /d₂.

Percentage of Thermal Shrinkage

The resulting plastic foam material was cut into various plate-shapedpieces measuring 200 mm long, 20 mm wide, 5 mm thick. The thus obtainedsmall plate-shaped plastic foam pieces were heated to 180° C. and drawnby about 10 percent to form pieces measuring about 220 mm long. The 220mm long, drawn plastic foam pieces were then charged in an oven andheated to 80° C. for 24 hours.

The length of the thermally shrunk, plastic foam material (1₁) wasmeasured in mm and the percentage of thermal shrinkage was calculatedfrom the Equation below:

    Percentage of Thermal Shrinkage=(100)×(220-1.sub.1)/(220).

Formability

The thus obtained plastic foam material was cut into 200 mm long, 200 mmwide, 5 mm thick, small, plate-shaped, samples.

Formability (the ability to mold the plastic foam material into plasticarticles) of the plate-shaped samples was determined by various methods.One method included the use of a cylindrical shaped object having acircular cross-section. The mouth of the cylindrical object includedflanges around the mouth. The bottom portions of the mouth was circularhaving an 8 cm diameter. This cylindrical object included apre-determined depth.

The surfaces of the aforementioned plate-shaped samples were heated to180° C. The respective mouths of each cylindrical member was covered bythe heated plate-shaped samples.

Using a circular column member with a 7 cm diameter, the plate-shapedsamples were pushed into the mouths of the cylindrical members. At atime just prior to breakage of the plate-shaped samples, a depth (h)(cm) of the circular column pushed inside the mouth of a cylinder-likemember was measured.

A drawing ratio defined by the equation below was obtained and used asthe index of formability. It was determined that a large drawing ratiowas indicative of superior formability.

    Drawing ratio=100×h/8

In Comparative Example 3, the blended resin composition exhibitedinsufficient shear viscosity. Generally, insufficient shear viscositycompromises the ability of the blended resin composition to producesignificant foam, during the foaming process. The inability to formfoam, can be attributed to the absence of a silane-modifiedpolypropylene resin. A lack of suitable shear viscosity suitable forforming foam, in turn, impairs the expansion ratio of the plastic foammaterial.

In Comparative Example 4, the prior art blended resin composition alsoexhibits insufficient shear viscosity. This insufficiency, in turn,imparts a low expansion ratio to the resulting plastic foam material.The insufficient shear viscosity and low foam production is due toexcess amounts of at least one of the two substantially incompatibleresins in the resin composition.

Similar to Comparative Example 4, Comparative Example 5 also indicates alow shear viscosity. The insufficient shear viscosity and its attendantlow foam production can be attributed to excessive amount of one of thepolyolefin (uncross-linked) thermoplastic resins such as polyethylene 1.

In Comparative Example 6, it seems apparent that there is a lack of auniform sea-and-island structure. This deficiency can be traced to thesimilar melt indexes of the two substantially incompatible thermoplasticresins, i.e., polyethylene 3 and polypropylene 2. The absence of thesea-and-island structure is believed to impart a low expansion ratio tothe plastic foam material.

In Comparative Example 7, even though the two substantially incompatiblethermoplastic resins are different from the one used previously inComparative Example 6, the overall effect on the plastic foam materialis similar to Comparative Example 6. The low expansion ratio of theplastic foam material is due to the similar melt indexes of the addedtwo substantially incompatible resins.

In Comparative Example 8, the absence of a uniform sea-and-islandstructure can be traced to the infinitesimal difference (less than 0.3 gper 10 minutes) between the melt indexes of the two substantiallyincompatible polyolefin based thermoplastic resins, exemplified bypolyethylene 1 and polypropylene 1. The absence of a uniformsea-and-island structure imparts a low expansion ratio to the plasticfoam material.

It seems clear from the test results that one of the two substantiallyincompatible thermoplastic resins can be finely and evenly dispersed inthe remaining one thermoplastic resin, forming a "micro" sea-and-islandstructure.

Because of its specific correlation with the two thermoplastic resins,the silane-modified thermoplastic resins also can be uniformly dispersedin the blended resin composition. The blended resin composition, aswhole, has a shear viscosity suitable for forming foam.

This in turn yields a plastic foam material with a high expansion ratio.The blended resin composition is characterized as having anuncross-linked portion, with improved fluidity. This feature, in turn,imparts superior formability properties to the plastic foam material.Furthermore, the blended resin composition of this invention isre-utilizable.

The portion with low cross-linking density can be re-melted. The portionwith high cross-linking density can be used as filling material. On theother hand, the amount of the silane-modified, thermoplastic resin addedis limited to less than 50 parts by weight per 100 parts of the blendedresin composition.

During the foaming process, the aforementioned limitation substantiallyreduces the undesirable inner stress caused by cross-linking.Accordingly, the plastic foam material obtained from such a blendedresin composition exhibits excellent dimensional stability.

Described hereinafter is embodiment 3 encompassing a method forproducing a plastic foam material which is light weight, and exhibitsimproved heat-insulating properties, has increased compression strengthand good resilience.

EMBODIMENT 3

For the third embodiment, a plurality of foamable tubes are used to makethe plastic foam material. The foamable tube includes an outer layer andan inner layer. The outer layer is composed of at least one of athermoplastic resin and a blended resin composition consisting of athermoplastic resin and a foaming agent.

The inner layer is composed a blended resin composition consisting of athermoplastic resin and a foaming agent.

The thermoplastic resin for use in the blended resin compositionincludes at least one of an olefin resin such as low-densitypolyethylene, high-density polyethylene, straight-chain low-densitypolyethylene, random polypropylene, homopolypropylene, blockpolypropylene; polyvinyl chloride, chlorinated polyvinyl chloride, ABSresin, polystyrene, polycarbonate, polyamide, polyvinylidene fluoride,polyphenylene sulfide, polysulfone, and polyether ether ketone.

The aforementioned resins can be used individually or in combinationwith others as copolymers. In order to substantially increase theresiliency of the resulting plastic foam material, it is desirable touse at least one olefin based resin such as low-density polyethylene,high-density polyethylene, straight-chain low-density polyethylene,random polypropylene, homopolypropylene, or block polypropylene. Amixture of the aforementioned resins is preferred.

In particular, a mixture containing at least one of high-densitypolyethylene or homopolypropylene is especially desirable.

The thermoplastic resins for use in the blended resin composition forforming the inner layer and the outer layer can be identical ordifferent. If different, the thermoplastic resins used in both must becapable of thermally adhesing to each other.

Examples of thermoplastic resins that satisfy the above condition ofthermo-adhesiveness include the following combinations: high-densitypolyethylene and low-density polyethylene, high-density polyethylene andstraight-chain low-density polyethylene, high-density polyethylene andhomopolypropylene.

In order to improve the resilience and strength of the resulting foambodies, it is desirable to include combinations exemplified byhigh-density polyethylene and low-density polyethylene, high-densitypolyethylene and straight-chain polyethylene, high-density polyethyleneand homopolypropylene. In particular, the combination of high-densitypolyethylene and homopolypropylene is especially desirable.

The thermoplastic resins can be cross-linked as needed. Cross-linkingeffectively increases the expansion ratio which, in turn, makes theresulting plastic foam material lighter.

Various methods can be utilized to achieve effective cross-linking ofthe respective components, such as (1) radiation, (2) use of a peroxidewhich is melt-mulled into a thermoplastic resin at a temperature belowthe decomposition point of the peroxide, or (3) melt-mulling asilane-modified thermoplastic resin. In (2) above, the resulting mixtureis then heated at a temperature above the decomposition point of theperoxide. In (3) above, the silane-modified thermoplastic resin ismelt-mulled together with a cross-linking catalyst specifically activeonly to the silane-modified thermoplastic resin to produce a plasticresin, followed by water-processing the mixture.

For purposes of this embodiment, the term "silane-modified thermoplasticresin" includes thermoplastic resins modified by an unsaturated silanecompound by graft polymerization. The silane-modified thermoplasticresin may include silane-modified thermoplastic polyethylene orsilane-modified thermoplastic polypropylene.

The unsaturated silane compound noted above can be generally expressedas

    R'SiR".sub.m Y.sub.3-m.

where

R' includes an organic functional group, e.g., an alkenyl group such asa vinyl group, an allyl group, a propenyl group or a cyclohexanyl group;a glycidyl group; an amino group; a mothacrylic group; a halogenatedalkyl group such as a γ-chloroethyl group or γ-bromoethyl group;

R" includes an aliphatic saturated hydrocarbon such as a methyl group,an ethyl group, a propyl group or a decyl group;

Y includes a hydrolytic organic functional group such as a methoxylgroup, an ethoxyl group, a formyloxyl group or a propionoxyallyl aminogroup;

"m" can be 0, 1 or 2.

Because cross-linking is quick, it is preferable to use an unsaturatedsilane compound generally expressed as:

    CH.sub.2 ═CHSi(OA).sub.2,

where

A includes an aliphatic saturated hydrocarbon with 1-8 carbon atoms,preferably from 1-4 carbon atoms. Examples of CH₂ ═CHSi(OA)₂ includevinyl trimethoxysilane, vinyl triethoxysilane and vinyltriacetoxysilane, etc.

The method for producing the silane-modified thermoplastic resin notedabove includes producing a silane-modified polyethylene from a reactionbetween polyethylene, an organic peroxide and a silane compound,expressed as

    R'SiR"Y.sub.2

where

R' includes at least one of an ethylene-like unsaturated group, an epoxygroup, or an amino group. The ethylene-like unsaturated group mayfurther include at least one of a vinyl group, allyl group, propenylgroup, cyclohexenyl group, γ-methacryloxypropyl group, etc;

Y is a hydrolytic organic groups, and

R" is either a R group or Y group as defined above.

In cases where the silane modified thermoplastic resin includes a silylgroup, such as methoxyl, hydrolysis with water will produce a hydroxylgroup.

The thus obtained hydroxyl group, in turn, can react with a hydroxylgroup of another molecule to create Si-O-Si bonds which in turn, aid incross-linking silane-modified thermoplastic resins. It is preferred thata silanol condensation catalyst be used to promote the cross-linkingreactions.

Any conventional method for adding silane-modified thermoplastic resinscan be utilized so long as the graft polymer is added uniformly.According to one method, a thermoplastic resin and silane-modifiedthermoplastic resin can be fed into a single-axis or twin-axis extruderand melt-mulled. Another method includes melt-mulling the respectiveconstituents with a roller. Yet another method utilizes a kneader tomelt-mull.

The step of water processing can be accomplished by immersing in water.Water processing can also be accomplished by exposure to steam, followedby processing at a temperature of 100° C. or greater. For temperaturesgreater than 100° C., pressurization is required.

If the water or steam temperature during water processing is too low,the cross-linking reaction proceeds too slowly. If the temperature istoo high, the outer layers fuse to each other. Therefore, a temperatureof from about 50° to about 130° C. is desirable, with 90° to about 120°C. being especially desirable.

If the duration of the water processing is too short, the cross-linkingreaction may not proceed to completion. If the duration is too long, theouter layers may fuse to each other. Therefore, a duration time of from5 minutes to about 12 hours is preferred.

If the amount of silane-modified thermoplastic resin added is too great,too much cross-linking will occur, and the resulting foam material willhave too low an expansion ratio. If the amount is too small, the foamcells burst, preventing generation of uniform cells. Therefore, about 5to about 50 parts by weight of the silane-modified thermoplastic resinto 100 parts by weight of thermoplastic resin is preferred, morepreferably from about 20 to about 35 parts by weight.

A cross-linking catalyst can be added to the resin composition to aid incross-linking the silane-modified thermoplastic resins. Suitablecross-linking catalysts are exemplified by one of a dibutyl tindiacetate, dibutyl tin dilaurate, dioctyl tin dilaurate, tin octanoate,tin oleate, lead octanoate, 2-ethyl hexane zinc, cobalt octanoate, leadnaphtenate, lead caprylate and zinc stearate.

If the amount of cross-linking catalyst added to 100 parts by weight ofthe thermoplastic resin exceed 1 part by weight, the expansion ratio ofthe resulting plastic foam material is decreased. Alternatively, if theamount of the cross-linking catalyst falls below 0.001 parts by weight,the rate of the cross-linking reaction is slowed thus requiringadditional water processing time.

It is preferable that the amount of the cross-linking catalyst be fromabout 0.01 to about 0.1 parts by weight.

The peroxide for use in the cross-linking reaction may include one of adibutyl peroxide, dicumyl peroxide, tert-butylcumyl peroxide,diisopropyl peroxide. In particular, dicumyl peroxide, tert-butylcumylperoxide are desirable, and dicumyl peroxide is especially desirable.

Adding too much peroxide facilitates the decomposition reaction of theresin, resulting in coloration of the resulting foam material. If thereis too little peroxide, the cross-linking in the thermoplastic resin isinadequate.

Therefore, for 100 parts by weight of the thermoplastic resin, fromabout 0.5 to about 5 parts by weight of peroxide is preferable and, morepreferably, from about 1 to about 3 parts by weight.

If radiation is used as a means to cross-link the thermoplastic resins,then excessive radiation causes excessive cross-linking to take place.In this event, the expansion ratio of the resulting plastic foammaterial decreases.

Alternatively, a very low radiation dosage causes the foam cells toburst, preventing uniform cell formation. Accordingly, a radiationdosage of from 1 to about 20 Mrad is desirable, with from about 3 toabout 10 Mrad being especially desirable.

Radiation may be applied in many ways, one of which may include usingtwo electron beam generators between which the plastic resin is passedto expose the two thermoplastic resins.

If necessary, a reinforcer such as short-fiber glass, short-fibercarbon, or short-fiber polyester or a filler such as calcium carbonate,aluminum hydroxide, or glass powder can be added to the plastic resin inorder to increase the strength of the plastic foam material.

If excessive amounts of the short-fiber glass is added as a reinforcer,cells get destroyed during foaming, thus lowering the expansion ratio.If too little short-fiber glass is added, the reinforcing strength ofthe plastic foam material is inadequate. Therefore, for 100 parts byweight of the thermoplastic resins, 1 to 20 parts by weight ofshort-fiber glass is desirable, and 3 to 10 parts by weight isespecially desirable.

If the short-fiber glass fibers are too long, the resulting plastic foammaterial becomes too heavy. If the short-fibers glass fibers are tooshort, the resulting plastic foam material is not adequately reinforced.Therefore, a length of from about 1 to about 30 mm is desirable, and alength of from 3 to about 5 mm is especially desirable.

When adding a filler, it is desirable to add from about 10 to about 100parts by weight of the filler to about 100 parts by weight of thethermoplastic resins. If too much filler is added, the resulting plasticfoam material is too heavy. If too little is added, the resultingplastic foam material is not adequately reinforced. It is preferable toadd from about 30 to about 50 parts by weight of the filler.

The foaming agent referred to above must be one which has adecomposition temperature that is higher than the melting point of thethermoplastic resins being used. Examples include inorganicthermodecomposing foaming agents such as sodium bicarbonate, ammoniumcarbonate, ammonium bicarbonate, azide compounds, or sodium borohydrideand organic thermodecomposing foaming agents such as azodicarbonamide,azobisisobutylnitrile, N,N'-dinitrosopentamethylene tetramine,P,P'-dinitrosopentamethylene tetramine, P,P'-oxy bisbenzenesulfonyl,hydrazide, barium azodicarbonate, or trihydradinotriazine.

Azodicarbonamide, which has a high gas yield, good hygienic propertiesand an easily adjustable decomposition point and decomposition rate, isdesirable.

In the present invention, high-foaming materials and low-foamingmaterials are relative labels referring to the expansion ratio of theplastic foam material. The material with the higher expansion ratio isreferred to as the high-foaming material, and the material with thelower expansion ratio is referred to as the low-foaming material.Generally, the inside layer is made up of a high-foaming materialrelative to the outer layer's low-foaming material.

The expansion ratio of the plastic foam material made from the foamcomposition can be adjusted through selection of the type ofthermoplastic resin, degree of polymerization, crystallization, densityof cross-linking, type and amount of foaming agent added. It isdesirable, from the point of view of ease of adjusting the expansionratio, to adjust the amount of the foaming agent. In this case, if theamount of the foaming agent is too large, cells break, resulting incells that are not uniform. This decreases the compression strength ofthe resulting plastic foam material. If the amount is too small, foamingdoes not occur. Therefore, for 100 parts by weight of the thermoplasticresin, it is desirable to add from about 1 to about 25 parts by weightof foaming agent.

More concretely, adjustments can be made by finding the gas yieldcorresponding to the desired expansion ratio, and a corresponding amountof foaming agent can be added.

In the case of high foaming materials, a high expansion ratio results ingreater radiative heat transfer, which increases the heat conductivityof the plastic foam material while simultaneously decreasing itsheat-insulating properties; whereas a low expansion ratio makes theresulting plastic foam material heavier. Therefore, an expansion ratioof from about 10 to about 50 is desirable. A expansion ratio of fromabout 25 to about 40 is especially desirable, and a expansion ratio offrom about 30 to about 40 is most desirable.

For low foaming materials, a high expansion ratio results in a lowercompression strength in the resulting plastic foam material. Therefore,an expansion ratio of 5 or lower is desirable, with an expansion ratioof 3 or lower being especially desirable.

Thus, it is preferable to adjust the amount of the foaming agent in highor low foaming materials so that the expansion ratio falls within theabove respective ranges.

The thermoplastic resin for use in the outer surface of the foamabletube can include the same thermoplastic resin as the one used for theplastic foam material described above.

It is desirable to use a thermoplastic resin for the outer surface. Athermoplastic resin has the added advantage of imparting improvedrigidity to the lattice-shaped or honeycomb-shaped resin walls. Thisfeature, in turn, significantly improves the compression strength of theresulting plastic foam material.

In the present invention, "honeycomb" refers to a structure wherehexagons are arranged continuously with shared segments.

The thermoplastic resin that forms the outer layer and the thermoplasticresin that forms the inner layer can be identical or different. If theyare different, they need to be thermo-adhesive (capable of fusing in thepresence of heat).

Examples of combinations of thermoplastic resins exhibitingthermo-adhesiveness are high-density polyethylene and low-densitypolyethylene, high-density polyethylene and straight-chain low-densitypolyethylene, high-density polyethylene and homopolypropylene, andpolyvinyl chloride and polyvinyl acetate.

As with the thermoplastic resin for use in the blended resincomposition, the thermoplastic resin for forming the outer layer can besupplemented with reinforcing agents such as short-fiber glass,short-fiber carbon, short-fiber polyester or with fillers such ascalcium carbonate, aluminum hydroxide, and glass powder.

The method of producing the foamable tube can involve, for example,feeding the thermoplastic resin and a foaming agent which forms theinner layer into a twin-axis extruder, melt-mulling at a temperaturelower than the decomposition point of the foaming agent, and extrudingthe result into a tube shape.

If the outer layer is composed of a blended resin composition, adifferent twin-axis extruder is fed with a thermoplastic resin and afoaming agent.

If the outer layer is a plastic resin compound, then the thermoplasticresin is fed into the twin-axis extruder. If necessary, melt-mulling isperformed at a temperature lower than the decomposition point of thefoaming agent. Then they are co-extruded so that the inner layer iscovered in a concentric fashion. The extruded tube is then cut toprescribed lengths. Foaming is done later.

Another method for producing foamable tubes is to feed the foaming agentand a thermoplastic resin into a twin-axis extruder. Melt-mulling isperformed at a temperature lower than the decomposition point of thefoaming agent. The product is then extruded in a tube shape and cut toprescribed lengths. Then, the thus obtained precut foamable tubes areimmersed in an organic solvent solution which dissolves the low-foamingresin material.

If the method for forming the foamable tubes includes the use of anorganic solvent solution, the organic solvent solution must be capableof dissolving the blended resin composition. The following are examplesof such solutions: aromatic hydrocarbons such as toluene and xylene,halogenated hydrocarbons such as methylene chloride and dichloromethane,and ketone compounds such as acetone and methyl ethyl ketone.

More specifically, if polystyrene or polyvinyl chloride is used as thethermoplastic resin in the blended resin composition, it is desirable touse methylene chloride as the organic solution. If polyvinyl chloride isused as the thermoplastic resin in the blended resin composition, it isdesirable to use methyl ethyl ketone as the organic solution.

If the thermoplastic resins used for the outer layer are very thick, theresulting plastic foam material is heavier. If the thermoplastic resinsare very thin, the compression strength of the resulting plastic foammaterial is decreased. Therefore, a thermoplastic resin ranging inthickness of 0.05 to about 5.0 mm is desirable, with 0.1 to about 2.0 mmbeing especially desirable.

It is preferred that the cross-section of the foamable tube be circularin shape, but in certain cases, it is acceptable to form ellipses andrectangles with rounded corners.

The present invention also involves a method for producing plastic foammaterial wherein a plurality of foamable tubes are arranged together, sothat during foaming, adjacent outer layers either fuse together, or byfoamable tubes being arranged in a staggered fashion so that thefoamable tubes touch each other.

The foamable tubes are supported by a pair of thickness-regulatingbodies in order to limit foaming in the axis-direction. The intervalbetween the thickness-regulating bodies is kept constant. The foamabletubes are heated and foaming takes place.

Whether the thickness-regulating bodies used in the present inventionare stationary or whether they can be moved in a certain direction isnot specified. For example, it is possible to use a pair of iron platesor iron meshes or a pair of belts that move at identical speeds inidentical directions. In terms of ease of assembly and mass production,the use of the belts described above is desirable.

The foamable tubes are supported by the thickness regulating bodiesdescribed above which limit foaming in the axis-direction. The foamabletubes between the thickness-regulating bodies are arranged so thatadjacent outer layers fuse together during foaming or are arranged in astaggered fashion so that the foamable tubes touch each other.

"Adjacent outer layers" refers to the outer layers of the foamable tubessurrounding one foamable tube. "A staggered arrangement" refers to 6foamable tubes arranged with a foamable tube at their center.

When the foamable tubes are arranged so that adjacent outer layers fusetogether, it is desirable to have at least one other foamable tubearranged so that it is touching. This is because the adjacent foamabletubes can prevent the outer layer from foaming or expanding toward theoutside, thereby giving inward foaming priority over outward foaming.This permits the inside of the resin or resin foam outer layer to befilled with the foam created by the inner layer. This provides improvedheat-insulating properties.

When the foamable tubes are arranged so that adjacent outer layers fusetogether, it is desirable to have the foamable tubes arranged so thatthey have fixed horizontal and vertical intervals. This is because theresin or resin foam forming the outer layer can form a lattice improvingthe compression strength of the resulting plastic foam material.

When the foamable tubes are placed in a staggered arrangement, the resinwalls or the resin foam formed from the outer layer takes on a honeycombstructure. Foaming in this manner provides improved compression strengthin the resulting plastic foam material compared to the arrangementinvolving adjacent outer layers fused together.

When the foamable tubes are arranged in a staggered arrangement, thefoamable tubes should preferably satisfy the equation below.Satisfaction of the equation improves the compression strength andinsulating properties of the resulting plastic foam material because thespace inside the outer layer is completely filled up during foaming.

    (Φ.sub.2 /Φ.sub.3).sup.2 ≦1-2√3(πTI),

where

Φ₂ is the inner diameter of the inner layer,

Φ₃ is the outer diameter of the inner layer, and

TI is the expansion ratio of the foam obtained from the inner layerafter foaming.

If a thermoplastic resin is used for the outer layer of the foamabletube, the outer layer becomes a resin wall having a honeycomb-shapedcross-section. This provides further improvements in compressionstrength as well as improved rigidity in bending strength.

In the present invention, the foamable tubes are supported on both endsby a pair of thickness-regulating bodies. Possible methods forsupporting the foamable tubes includes arranging the foamable tubes onthe first thickness-regulating body and then stacking the secondthickness-regulating body on top.

An alternative method includes arranging the foamable tubes on a movingthickness-regulating body located at the bottom. This is then fed to topand bottom thickness-regulating bodies. In this method, the intervalbetween the thickness-regulating bodies must be adjusted to the heightof the foamable tubes.

When foamable tubes are arranged with their adjacent outer surfacesfused together, it is possible to use a rotating roller with holes forfoamable tubes arranged around the perimeter so the tubes have aperpendicular arrangement. This aids in melting and fusing the adjacentouter.

Foamable tubes are supplied from the top and foamable tubes fallstraight down and are arranged on the moving thickness-regulating body.

When the foamable tubes are arranged in a manner wherein each tubescontact at least one other tube, the above feeding method can still beused. In this case, the rotating roller sends foamable tubes to avibrating feed plate having side walls arranged to the left and rightoriented downward and diagonal toward the moving thickness-regulatingbodies. Then the foamable tubes are sent from the feed plate to thethickness-regulating bodies.

When the foamable tubes are placed in a staggered arrangement, the abovemethod can be used. In this case, the side walls of the feed plate aretapered narrowing toward the thickness-regulating bodies.

In order to improve the compression strength of the resulting plasticfoam material, it is desirable to use a flat reinforcing materialbetween the thickness regulating bodies and the foamable tubes. The flatreinforcing material is exemplified by glass paper, a chopped strandmat, a metal sheet, a thermoplastic resin sheet or a thermosetting resinsheet.

If the glass paper or the fibers in the glass paper used are too heavy,the resulting plastic foam material is too heavy, and if they are toolight, the resulting plastic foam material is too weak. Therefore, glasspaper, including one with fibers in it, should weigh from about 10 toabout 100 g/m² and preferably from about 20 to about 50 g/m².

The thermoplastic resin for use in the thermoplastic resin sheetdescribed above include at least one of polyethylene, polypropylene andpolyethylene terephthalate. In order to improve adhesion between theplastic resin sheet and the plastic foam material, it is desirable touse the same type of thermoplastic resin as the one used to form theplastic foam material.

The thermosetting resin for use in the thermosetting resin sheet caninclude melamine resin, phenol resin, epoxy resin or unsaturatedpolyester.

The metal to be used in the metal sheet may include aluminum or iron.

If the plastic resin, thermosetting resin, or metal sheet describedabove is too thick, the resulting plastic foam material becomes tooheavy. If it is too thin, the resulting plastic foam material is notadequately reinforced. Accordingly, the plastic resin, thermosettingresin, or metal sheet should range in thickness from about 0.05 to about1 mm. and preferably from about 0.1 to about 0.5 min.

In the present invention, when the foamable tubes are supported withthickness-regulating bodies, heating is performed while maintaining aconstant interval between the thickness-regulating bodies. The heatingmethod is not particularly specified as long as the temperature goesabove the decomposition point of the foaming agent forming the foamingmaterial. Examples include electric heaters, infrared heaters, andheating devices that circulate a heating medium such as oil or air.

The method for producing foam bodies of the present invention is asdescribed above.

The foamable tubes of the present invention have an internal hollow areain the axis direction. Therefore, when heat is applied to the foamabletube, heat goes not only to the outside but also to the inside of thefoamable tube through the hollow area. This makes uniform foamingpossible and prevents the need for overheating the outside in order toheat the inside. This keeps the outside foam from bursting and maintainsthe lightness and compression strength of the resulting plastic foammaterial.

When the foamable tubes are arranged so that adjacent outer layers fusetogether, the outer layer, made from a low-foaming material or a plasticresin, forms a plastic wall that provides a plastic foam material withimproved compression strength.

Furthermore, when the foamable tubes are arranged so that the outerlayers fuse together and each foamable tube is in contact with at leastone other foamable tube, the plastic foam material fills the inside ofthe resin wall formed by the outer layer of the resin foam wall. Thisimproves heat-insulation properties.

Also, when the foamable tubes are arranged so that the outer layers fusetogether and the foamable tubes are arranged so that they have fixedhorizontal and vertical intervals, the resin walls or the resin foamwalls resulting from the outer layers form a lattice-shapedcross-section. This provides a plastic foam material with improvedcompression strength compared to the arrangement where the outer layersfuse together and each foamable tube is in contact with at least oneother foamable tube.

If a staggered arrangement is used, the outer layers form resin walls orresin foam walls have a honeycomb-shaped cross-section. Therefore, theresulting plastic foam material has improved compression strength aswell as bending strength as compared to the arrangement described abovewhere the outer layers fuse together.

In cases where the foamable tubes are arranged in a staggeredarrangement, provided the foamable tubes satisfy equation below, theresin walls or the resin foam walls resulting from the outer layers arefilled with foam from the inner layers, thus improving heat-insulationproperties.

    (Φ.sub.2 /Φ.sub.3).sup.2 ≦1-2√3/(πTI)

where,

Φ₂ is the inner diameter of the inner layer,

Φ₁ is the inner diameter of the inner layer, and

TI is the expansion ratio of the foam obtained from the inner layerafter foaming.

Finally, since the inner layers foam to become a high-foaming plasticfoam material, a light plastic foam material is obtained.

According to this embodiment, a 50 mm single-axis extruder is fed withthe materials to make up the inner layer: 50 parts by weight of ahigh-density polyethylene exemplified by Mitsubishi Petrachemicalsproduct name EY40H, 50 parts by weight of a polypropylene exemplified byMitsubishi petrachemicals product name PY230, 20 parts by weight of asilane-modified thermoplastic polypropylene exemplified by MitsubishiPetrachemicals product name XPM800H, and 8 parts by weight of anazodicarbonamide.

These are melt-mulled at 180° C. and extruded in a tube shape. At thesame time, the following materials making up the outer layer are fed toa 25 mm single-axis extruder in the amounts specified in Table 4: ahigh-density polyethylene exemplified by Mitsubishi Petrachemicalsproduct name EY40H, a polypropylene exemplified by MitsubishiPetrachemicals product name PY230, a silane-modified thermoplasticpolypropylene exemplified by Mitsubishi Petrachemicals product nameXPM800H, and an azodicarbonamide. These are melt-mulled at 180° C. andare co-extruded using a crosshead die. After cutting into 25 mm lengths,the product is immersed in water at a temperature of 98° C. for onehour.

Referring to FIG. 1, this results in a foamable tube 3 comprising aninner layer 1 and an outer layer 2. The thickness and inner diameter ofthe inner layer as well as the thickness of the outer layer areindicated in Table 4.

In the examples, described below, foamable tube 3 is used to produce aplastic foam material.

EXAMPLE 26 AND 27 OF TABLE 4

In the figures, "front" refers to the rightward direction.

Referring to FIG. 2, plastic foam material 3, produced as describedabove, is fed to a hopper 4.

An exit hole 5 is arranged at a bottom of hopper 4 having a shapecorresponding to cavities 6 arranged on a circumference of a 200 mmradius rotating roller 7. Rotating roller 7 is near exit hole 5.

Referring to FIG. 3, cavities 6 arranged on the circumference ofrotating roller 7 have a shape that corresponding to foamable tube 3,and are arranged at 100 mm intervals in the axis direction and 157 mmintervals along the circumference. FIG. 3 shows only one part ofcavities 6.

Referring again to FIG. 2, rotating roller 7 rotates in acounterclockwise direction. When cavity 6 is directly below exit hole 5,foamable tubes 3 in hopper 4 drop into cavity 6. A rear extension 19b ofa tetrafluoroethylene lower continuous moving belt 19 is located belowrotating roller 7 at a distance roughly equal to the height of plasticfoam material 3. When rotating roller 7 rotates and cavity 6 ispositioned straight down, foamable tubes 3 are placed onto rearextension 19b. Continuous moving belts 18, 19 are arranged in front ofrotating roller 7 at a distance equal to the height of foamable tube 3.

An upper continuous moving belt facing side 18a and a lower continuousmoving belt facing side 19a act as thickness-regulating bodies. Thefoamable tubes 3 dropped from rotating roller 7 are arranged andsupported naturally between continuous moving belt facing sides 18a, 19aby continuous moving belt 19, located below rotating roller 7. A heatingdevice, not shown in the figures, applies 230° C. for 7 minutes, causingthe blended resin composition to foam.

Referring to FIG. 4, a sheet-shaped plastic foam material 12 is formedby the fusion of foam from inner layer 1 within a resin wall 14resulting from the metamorphosis of outer layer 2 of tubes 3.

Table 4 shows the results of measuring the following properties of theresulting plastic foam material: the expansion ratios as observed fromthe inner layer and the outer layer of the plastic foam material, thethickness of the plastic foam material, 25 percent compression strength,compression-permanent-setting, and heat conductivity. The measurementswere made according to the methods described below.

The expansion ratios were measured according to JIS K6767. The thicknessof the plastic foam material was measured by measuring ten arbitrarypoints on the plastic foam material and calculating the average. The 25percent compression strength was measured according to JIS K6767. Thecompression-permanent-setting was measured according to JIS K6767. Heatconductivity was measured according to JIS A1418.

EXAMPLES 28 AND 29 OF TABLE 4

Referring to FIGS. 5-7, plastic foam material 3, produced as describedabove, is fed to a hopper 4. An exit hole 5 at a bottom of hopper 4 hasa shape corresponding to cavities 6 on a circumference of a 200 mmradius rotating roller 7. Rotating roller 7 is near exit hole 5.Cavities 6 on the circumference of rotating roller 7 have a shape thatcorresponding to foamable tube 3, and are spaced at 100 mm intervals inthe axis direction and 157 mm intervals along the circumference. Cavity6 has a rectangular shape, and exit hole 5 of hopper 4 has acorresponding shape. Only 2 cavities are depicted, but it is possible toarrange 3 or more cavities as well.

Rotating roller 7 rotates in a counterclockwise direction. When cavity 6is directly below exit hole 5, foamable tubes 3 in hopper 4 drop intocavity 6. Vibrating feed plate 40 slopes forward and downward betweenrotating roller 7 and a rear extension 11b of a tetrafluoroethylenecontinuous moving belt 11. Feed plate 40 is arranged so that thedistance between the rear part of feed plate 10 and rotating roller 7 isroughly equal to the height of foamable tube 3, and the front part offeed plate 40 is arranged so that it is positioned above rear extension11b. Side walls 41 are on both ends of feed plate 40.

Continuous moving belts 10, 11 are arranged in front of feed plate 40.An upper continuous moving belt facing side 10a and a lower continuousmoving belt facing side 11a act as thickness-regulating bodies. Thefoamable tubes 3 dropped from rotating roller 7 are arranged andsupported naturally between continuous moving belt facing sides 10a, 11aby continuous moving belt 11, located below rotating roller 7. Whenfoamable tubes 3 are sent from rotating roller 7 to feed plate 40, theweight of foamable tubes 3 as well as the vibrations of feed plate 40cause foamable tubes 3 to move forward on feed plate 40. The vibrationsof feed plate 40 is produced by conventional means and is therefore notshown, nor further described. Foamable tubes 3 are placed on rearextension 11b arranged in such a way that each foamable tube is incontact with at least one other foamable tube. A conventional heatingdevice, not shown in the figures, applies 230° C. for 7 minutes, causingthe blended resin composition to foam.

Table 4 shows the results of measuring the following properties of theresulting plastic foam material: the expansion ratio of the foam fromthe inner layer and the outer layer of the plastic foam material, thethickness of the resulting plastic foam material, 25 percent compressionstrength, compression-permanent-setting and heat conductivity. Themeasurements were made using the same methods as above.

EXAMPLES 30-33 OF TABLE 4

Referring to FIGS. 8 and 9, a plastic foam material as shown in FIG. 9was obtained in the same manner as in Example 28 of Table 4 except thata feed plate 8, tapered toward the front or downward end, is usedinstead of feed plate 40.

When foamable tubes 3 move forward due to their own weight and thevibrations of feed plate 8, side walls 9 on feed plate 8 guide foamabletubes 3 toward the center line of feed plate 8 because of the taperingof feed plate 8.

Referring to FIG. 10, using feed plate 8 permits foamable tubes 3 to bearranged in a stable staggered fashion with each foamable tube touchingat least one other foamable tube.

Arrangement B of Table 4 shows the results of measuring the followingproperties of the plastic foam material produced by using feed plate 8:the expansion ratio of the foam obtained from the inner layer and theouter layer, the thickness of the plastic foam material body, 25 percentcompression strength, compression permanent setting and heatconductivity. The measurements were made using the same methods asabove.

COMPARATIVE EXAMPLES 11-13 OF TABLE 4

A 50 mm single-axis extruder is fed with the following materials: 50parts by weight of high-density polyethylene exemplified by MitsubishiPetrochemicals product name EY40H, 50 parts by weight of polypropyleneexemplified by Mitsubishi Petrochemicals product name PY 230, 20 partsby weight of silane-modified thermoplastic polypropylene exemplified byMitsubishi Petrochemicals product name XPM800H, and 8 parts by weight ofazodicarbonamide. This mixture is melt-mulled at 180° C., extruded in atube shape, and cut to 25 mm lengths. The foamable tubes are completedafter immersion in water for an hour at 98° C.

Plastic foam material were produced in the same way as in examples 26and 28 using single layer foamable tubes instead of two-layer foamabletubes.

Table 4 shows the results of measuring the following properties of theresulting plastic foam material: the expansion ratio of the foam createdby the inner layer and the outer layer of the plastic foam material, thethickness of the plastic foam material, 25 percent compression strength,compression permanent setting and heat conductivity. The measurementswere made according to the same methods as used above.

Referring to Table 4, an arrangement of mutually touching foamable tubesarranged in a staggered fashion provides a better compression strengthin the resulting plastic foam material compared to an arrangement whereadjacent outer layers fuse together.

The method of producing plastic foam material of the present inventionis described above. According to the present invention, it is possibleto provide plastic foam materials having lattice-shaped orhoneycomb-shaped cross sections, superior rigidity in compressionstrength, bending strength, and, because foam is present within theresin walls, superior heat-insulating properties and lightness.

A porous foam plate made from a plurality of foamable tubes 3 accordingto the method of the present invention has good permeability for water,air, and sound.

                                      TABLE 4                                     __________________________________________________________________________                                                     Comparative                                   Example                         Example                      Resin forming the outer layer                                                                  26  27  28  29  30  31  32  33  11  12   13                  __________________________________________________________________________    High-density polyethylene                                                                      --  50  --  50  --  50  --  50  --  --   --                  Polypropylene    100 50  100 50  100 50  100 50  --  --   --                  Silane graft polyethylene                                                                      --  20  --  20  --  20  --  20  --  --   --                  Azodicarbonamide --  1   --  1   --  1   --  1   --  --   --                  Inner diameter inner layer (mm)                                                                36  36  36  36  37.5                                                                              37.5                                                                              36  36  36  36   36                  Outer diameter inner layer (mm)                                                                38  38  38  38  38  38  38  38  38  38   38                  Thickness of outer layer (mm)                                                                  1   1   1   1   1   1   1   1   --  --   --                  Expansion ratio of outer layer                                                                 --  2.5 --  2.5 --  2.5 --  2.5 --  --   --                  Expansion ratio of inner layer                                                                 20  19  20  19  20  20  20  19  20  20   20                  Thickness (mm)   25  25  25  25  25  25  25  25  25  25   25                  25% compression strength (kg/cm.sup.2)                                                         3.86                                                                              3.23                                                                              4.20                                                                              3.64                                                                              4.87                                                                              4.04                                                                              5.02                                                                              4.83                                                                              1.40                                                                              2.38 2.47                Compression-permanent-setting                                                                  7.4 6.8 7.1 7.0 7.0 6.9 7.3 7.1 6.5 6.7  6.9                 Heat Conductivity                                                                              0.04                                                                              0.03                                                                              0.04                                                                              0.03                                                                              0.07                                                                              0.05                                                                              0.04                                                                              0.03                                                                              0.03                                                                              0.03 0.03                Arrangement of foam tubes                                                                      A   A   A   A   B   B   B   B   A   A    B                   __________________________________________________________________________     Arrangement A is arrangement where outer layers fuse together.                Arrangement B is staggered arrangement with tubes touching each other.   

The following is a description of preferred examples of the presentinvention, described in comparison with the comparative examples.

EXAMPLES 34 AND 35 OF TABLE 5

A blended resin composition is produced using a tumbler to mix 2.25parts by weight of a high-density polyethylene exemplified by MitsubishiPetrochemical product name PY20A, 2.30 parts by weight of a high-densitypolyethylene exemplified by Mitsubishi Petrochemical product name EY40H,2.40 parts by weight of a homopolypropylene exemplified by MitsubishiPetrochemical product name MA3, 0.25 parts by weight of a blockhomopolypropylene exemplified by Mitsubishi Petrochemical product nameBC4, and 1.9 parts by weight of a silane-modified polypropyleneexemplified by Mitsubishi Petrochemical product name Linklon XPM800HM.Then, a high-foaming blended resin composition is obtained by mixing 5parts by weight of this blended resin composition and 0.30 parts byweight of a thermodecomposing foaming agent azodicarbonamide exemplifiedby Otsuka Chemical product name Uniform AZ SO-20.

Next, the blended resin composition and the high-foaming resin materialare fed to two separate twin-axis extruders, melt-mixed at 180° C., andco-extruded to form a two-layer tube, which is cut every 30 mm.Referring to FIG. 1, the result is tube 3 having inner layer 1comprising a high-foaming blended resin composition and outer layer 2comprising a blended resin composition.

Referring to FIGS. 5 and 8, the resulting plurality of tubes 3 aredeposited in hopper 4. Hopper 4 has wide exit hole 5. Rotating roller 7having wide cavities 6 for holding tubes extending in the axis-directionand arranged symmetrically on the circumference is disposed below hopper4 and near exit hole 5. An aperture of wide cavity 6 has a shapematching the shape of exit hole 5 of hopper 4. Rotating roller 7 turnsin a counterclockwise direction. When cavity 6 comes directly below exithole 5, a plurality of tubes 3 in hopper 4 fall into cavity 6 in a row.

Vibrating feed plate 8 is inclined frontward and downward below rotatingroller 7. Feed plate 8 is shaped so that its width tapers from back tofront, with side walls 9 arranged vertically on both side edges.

Referring also to FIG. 6, when rotating roller 7, holding a row of tubesin cavity 6, rotates 180 degrees, the opening of cavity 6 is turneddownward so that tubes 3 inside cavity 6 fall onto feed plate 8 in onerow. Instead of two cavities 6, it is possible to use three or morecavities 6 arranged radially. By the time tubes 3 reach the exit of feedplate 8, they are arranged in a stable staggered formation due to sidewalls 9 moving tubes 3 toward the center line of feed plate 8 and due tothe vibrations of feed plate 8.

Continuous drive belts 10, 11 having thicknesses of 1 mm and separatedby an distance roughly equal to the height of tube 3 are near the exitof feed plate 8. The exit of feed plate 8 is located near an uppersurface of rear extension 11b. The two continuous belts 10, 11 serve asregulating members to restrict foaming in the axis direction duringheat-foaming, with opposing surfaces 10a, 11a of continuous belts 10, 11serving as the regulating surfaces for tubes 3. Opposing surfaces 10a,11a are preferably moved at an identical speed from back to front.

Tubes 3, arranged in a staggered formation, move from the tip of feedplate 8 onto rear extension upper surface 11b of lower continuous belt11 due to the vibrations of feed plate 8 as well as gravity. Referringto FIG. 10, the plurality of foamable tubes 3 on rear extension uppersurface 11b are arranged in a staggered formation so that they toucheach other. With the movement of upper and lower continuous belts 10,11, tubes 3, arranged in the manner described, are supported naturallyby the two opposing surfaces 10a, 11a.

Tubes 3 are heated for 30 minutes at 210° C. by an electric heater notshown in the drawings. The foaming blended resin composition foams sothat each tube 3 expands both inward and outward, thus filling the gapsbetween the tubes. Referring to FIG. 11, each tube 3 changes into ahexagonal tube 13 having a center hole 22 passing through the resultingplastic foam material. The outer layers fuse together, thus making aone-piece porous foam plate 24. Hexagonal shaft 23 has an outer layer 25and an inner layer 26. Table 5 shows the inner diameter of the innerlayer and the outer diameter of the outer layer for examples 34 and 35.

COMPARATIVE EXAMPLE 14

Referring to Table 5, a porous foam plate was obtained in the same wayas in example 34 except that the inner diameter of the outer layer ofthe two-layer tube is different from examples 34 and 35.

EXAMPLES 36 AND 37 OF TABLE 5

Two blended resin compositions were obtained by mixing 4 parts by weightof the blended resin composition from example 34 and 0.04 and 0.24 partsby weight of the foaming agent.

Next, the blended resin compositions are fed to two separate twin-axisextruders so that the low foaming plastic resin compound obtained fromthe mixture using 0.04 parts by weight of thermodecomposing foamingagent forms the outer layer, and co-extrusion takes place. Other thanwhat is described above, the examples obtain a porous foam plate in thesame manner as in example 34. The inner diameter of the inner layer andthe inner diameter of the outer layer of the tubes are as shown in FIG.1.

COMPARATIVE EXAMPLE 15

A porous foam plate is obtained in the same manner as example 34 exceptthat, as shown in Table 5, the inner diameter of the outer layer of thetwo-layer tube is different from examples 36 and 37.

A square of the porous foam plate from each of the examples was fixed ina container at a height just above the edge of the container so that theperimeter of the square was flush with the inner surface of thecontainer. Water was poured from a height 20 cm above the top surface ofthe porous foam, and the amount of water falling to the bottom of thecontainer in five minutes was measured. The results, and the diameter ofthe center holes in the different examples, are indicated in Table 5.

Referring to FIG. 12, porous foam plate 24 used in a square concretesewage pipe 17 is shown. Wide rectangular windows 28 are arranged atprescribed length-wise intervals on both sides of sewage pipe 17. Aporous foam plate 24 having a size matching the aperture area is fittedand fixed in each window 28. Because porous foam plate 24 has aplurality of holes 22 therethrough, when water is poured into sewagepipe 17 to a fixed level or higher, water flows from sewage pipe 17 tothe surrounding earth naturally and gradually through holes 22, thuspreventing sewage pipe 17 from filling up with water.

Porous foam plates according to the method of the present invention havegood permeability for water, air, and sound. Thus, they can be used inways other than the one for sewage pipe 17 described above as well.

In the porous foam plate obtained by the method of the presentinvention, the outer layers of each of the transformed hexagonal tubeforms a honeycomb structure of plastic resin or a low-foaming bodyhaving a expansion ratio of 3 or less. This results in good rigidity.The part inside the honeycomb structure is a high-foaming body with aexpansion ratio of 5 or more. This increases the overall lightness ofthe porous foam plate. Also, because each transformed hexagonal tube hasa hole therethrough with a diameter of from 1 to about 10 mm, the porousfoam plate itself has a plurality of holes with diameters of from 1 toabout 10 mm in an orderly arrangement at fixed intervals, thus making itappropriate for sewage pipes, as described above. Furthermore,production is easy and efficient.

EMBODIMENT 4

According to the fourth embodiment, a plastic foam material is formedfrom a thermoplastic core foam material (hereinafter referred to as"core material") which is integrally placed within each lattice orhexagonal space of a thermoplastic cover foam material (hereinafterreferred to as "cover material") having a cross-section with a latticeor honeycomb shape. The cover material includes at least one of a covermaterial and a thermoplastic resin.

The expansion ratio of the core material is from about 10 to about 50,while the expansion ratio of the cover material is lower than theexpansion ratio of the core material, with the difference of theexpansion ratios being at least 5 or greater. The present embodiment canalso include thermoplastic core foam materials formed integrally in eachlattice or hexagonal space of a thermoplastic resin body havingcross-section with a lattice or honeycomb shape.

The cover material, the core material and the thermoplastic resin bodyabove include a thermoplastic resin which can be used individually or incombination with other resins. Such a thermoplastic resin includes atleast one of an olefin resin such as low-density polyethylene,high-density polyethylene, straight-chain low-density polyethylene,random polypropylene, homopolypropylene, block polypropylene, polyvinylchloride, chlorinated polyvinyl chloride, ABS resin, polystyrene,polycarbonate, polyamide, polyvinylidene fluoride, polyphenylenesulfide, polysulfone, polyether ether ketone, or copolymers thereof.

It is desirable to use one or a mixture of an olefin resin such aslow-density polyethylene, high-density polyethylene, straight-chainlow-density polyethylene, random polypropylene, homopolypropylene, orblock polypropylene as the thermoplastic resin in the cover material,the core material, and the thermoplastic material because the finalplastic foam material will exhibit improved resiliency. In particular,high-density polyethylene and homopolypropylene are especiallydesirable.

Identical or different thermoplastic resins can be used for the covermaterial and the core material, as well as the resin body and the corematerial.

The thermoplastic resins used in the cover material and the corematerial can be cross-linked if necessary. Cross-linking is advantageousbecause it improves the expansion ratio and allows the final plasticfoam material to be light.

It is possible to add a reinforcing agent such as short-fiber glass,short-fiber carbon, or short-fiber polyester or a filler such as calciumcarbonate, aluminum hydroxide, or glass powder to the thermoplasticresins used in the cover material, the core material, and the resin bodyin order to improve the strength of the plastic foam material.

When short fibers are added as a reinforcer, too much fiber can causecell destruction during foaming, thus preventing a high expansion ratio.Very little fiber content can substantially decrease the reinforcingeffect on the plastic foam material.

Therefore it is desirable to use from about 1 to about 20 parts byweight of the short fiber per 100 parts by weight of the thermoplasticresins used in the cover material, the core material, and the resinbody. Using from about 3 to about 10 parts by weight is especiallydesirable.

The length the of the short fibers is similar to the previousembodiments.

When adding a filler, it is desirable to add from about 10 to about 100parts by weight of the filler per 100 parts by weight of thethermoplastic resins for use in forming the cover material, the corematerial and the thermoplastic resin.

If too much filler is added, the resulting plastic foam material is tooheavy. If too little is added, the resulting plastic foam material isinadequately reinforced. It is preferable to add from about 30 to about50 parts by weight of the filler.

An increase in the expansion ratio of the core material causes asubsequent increase in the radial heat transfer of the plastic foammaterial which, in turn, increases heat conductivity of the plastic foammaterial while decreasing its insulating properties.

If the expansion ratio of the core material is too low, the resultingplastic foam material is too heavy. Therefore the expansion ratio of thecore material should be from about 10 to about 50, more preferably fromabout 10 to about 50, and most preferably from about 15 to about 40. Aexpansion ratio of from 15 to about 30 is especially desirable.

If the core material is too thick, the plastic foam material issubstantially weakened. If it is the core material is very thin, theresulting plastic foam material becomes heavy. Therefore a thickness offrom about 10 to about 200 mm is desirable, and preferably from about 20to about 100 mm.

The thickness of the core material does not need to be uniform. It canbe non-uniform. The thickness of the core material referred to hererefers to the maximum thickness measured along a cross section.

The expansion ratio of the cover material should be lower than theexpansion ratio of the core material. The difference between theexpansion ratios should be at least 5 or greater. However, if thedifference between the two materials is less than 5, the strength of theplastic foam material is compromised, resulting in a weak foam material.

Therefore, it is preferred that the differences in the expansion ratiosbetween the core and the cover materials be from about 5 to about 39,and more preferably from about 15 to about 29.

If the cover material is too thick, the plastic foam material is heavy,while if it is too thin, the strength of the plastic foam material istoo low. Therefore the cover material should measure from about 0.05 toabout 5 mm in thickness, and more preferably from about 0.1 to about 2mm.

If the thermoplastic resin body is too thick, the plastic foam materialis heavy, while if it is too thin, the strength of the plastic foammaterial is low. Therefore the resin body should measure from about 0.05to about 30 mm in thickness, and more particularly from about 0.1 toabout 2.0 mm.

The thickness of the cover material and the resin body can be eitheruniform or non-uniform. The thickness of the cover material and thethermoplastic resin body referred to here refers to the averagethickness of the walls forming the lattice or honeycomb along the crosssection of the plastic foam material.

If the proportion of the cover material in the plastic foam material istoo high, the plastic foam material is too heavy. If it is very low, thestrength of the plastic foam material decreases. Therefore the amount ofthe cover material in the plastic foam material should be from about 10to about 50 parts by weight, with 10 to about 30 parts by weight beingpreferred.

For similar reasons, it is desirable that the proportion of thethermoplastic resin body in the plastic foam material be from about 5 toabout 35 parts by weight, with from about 8 to about 25 parts by weightbeing preferred.

The plastic foam material of the present embodiment comprises a covermaterial and a core material, or a thermoplastic resin body and a corematerial wherein a cross section of the cover material and thethermoplastic resin body forms a lattice shape or a honeycomb shape. Theuse of a thermoplastic resin body or a honeycomb shape is advantageousbecause it improves rigidity in the thickness-direction of the plasticfoam material. The shapes of the spaces in the lattice do not have to beidentical.

In the present embodiment, honeycomb refers to a cross-section havingthe shape of cells in a bee hive. Specifically, it refers to hexagonalshapes extended in two dimensions so that each side of each of thehexagons are shared with other hexagons. The hexagons do not have to beidentical in shape, and do not have to have to be equilateral as long asthey can be identified as hexagons by sight.

The plastic foam material of the present embodiment comprising a covermaterial and a core material, or a thermoplastic resin body and a corematerial, is generally formed as a sheet. However, the method ofintegrating the cover material and the core material, or thethermoplastic resin body and the core material, is not particularlyspecified.

Thermal fusion is generally used, but an adhesive material can also beused. In thermal fusion, if different thermoplastic resins are used forthe cover material and the care material, or the thermoplastic resinbody and the core material, the thermoplastic resins must be capable ofbeing thermally fused to each other.

The following combinations are examples of suitable thermoplastic resinswhich can be thermally fused with one another: high-density polyethyleneand low-density polyethylene, high-density polyethylene andstraight-chain low-density polyethylene, high-density polyethylene andhomopolypropylene, and polyvinyl chloride and polyvinyl acetate.

The following combinations are desirable because they improve thestrength and resilience of the final plastic foam material: high-densitypolyethylene and low-density polyethylene, high-density polyethylene andstraight-chain low-density polyethylene, and high-density polyethyleneand homopolypropylene. In particular, the combination of high-densitypolyethylene and homopolypropylene is especially desirable.

The polymer for use in forming the adhesive material must be capable ofeffectively joining the cover material and the core material or thethermoplastic resin and the core material. Suitable polymers for theadhesive material include ethylene vinyl acetate, ethylene vinylchloride copolymer, a copolymer of the thermoplastic resin monomersforming the cover material and the core material, and a copolymer of thethermoplastic resin monomers used for forming the thermoplastic resinbody and the core material.

To improve the adhesive properties of the adhesive, a copolymer of themonomers from the thermoplastic resin used for forming the covermaterial, the core material, or the thermoplastic resin body ispreferred.

The method of making the "plastic foam material" of the presentembodiment may include one of the following methods:

(1) Using a plurality of foamable pieces, wherein each one of theplurality of foamable pieces have identical heights and are arranged ona first thickness regulating body. Each one of the plurality of foamablepieces is positioned such that its lower surfaces touches the firstthickness regulating body. This allows the adjacent cover materials tofuse together. When foaming takes place, adjacent cover materials fusetogether. Upon foaming, the adjacent cover materials fuse together.

Identical pieces which have the same height comprise: (a) acolumn-shaped core material made of a high-foaming resin containing athermoplastic resin and a foaming agent, and (b) one of a low-foamingresin cover material containing a thermoplastic resin and a foamingagent and a resin cover material containing a thermoplastic resincovering at least a side-surface of the column-shaped core material.

This plurality of shaped foam pieces are used to form a stacked bodywhich is created by arranging a second thickness regulating body on top.The body is heated and foamed while a fixed interval between thethickness regulating bodies is maintained.

(2) Using a plurality of shaped foam pieces having the same height whichcomprises: (a) a column-shaped core material made of a high-foamingresin containing a thermoplastic resin and a thermodecomposing foamingagent, and (b) a low-foaming resin cover material containing athermoplastic resin and a thermodecomposing foaming agent or a resincover material containing a thermoplastic resin covering the entiresurface of the column-shaped core material. This plurality of shapedfoam pieces is arranged on a first thickness regulating body so thatthey do not overlap and so that adjacent cover materials fuse togetherduring foaming. A stacked body created by arranging a second thicknessregulating body on top is heated and foamed while a fixed intervalbetween the thickness regulating bodies is maintained.

(3) Using a plurality of shaped foam pieces having the same height whichcomprises: (a) a column-shaped core material made of a high-foamingresin containing a thermoplastic resin and a thermodecomposing foamingagent, and (b) a low-foaming resin cover material containing athermoplastic resin and a thermodecomposing foaming agent or a resincover material containing a thermoplastic resin covering at least theside-surface of the column-shaped core material. This plurality ofshaped foam pieces is arranged on a first thickness regulating body sothat one shaped foam piece is arranged for each point and inner area ofhexagon in a honeycomb shape which is hypothetically arranged on thefirst thickness regulating body. The arrangement is such that bottomsurfaces of the foam pieces touch the first thickness regulating body,and adjacent cover materials fuse together during foaming.

A stacked body is created by arranging a second thickness regulatingbody on top. The stacked body is heated and foamed while a fixedinterval between the thickness regulating bodies is maintained.

The three methods described above are preferred, but in certain cases,the following method is also used:

(4) A core material, a cover material, and a thermoplastic resin bodyare prepared beforehand. The core material is covered with the covermaterial or covered with the thermoplastic resin using thermal fusion oran adhesive to form a plastic foam material.

Method (3) must be used if a plastic foam material is to be made suchthat the cross-section of the cover material and the resin body forms ahoneycomb shape.

In the methods described above, the high-foaming resin core material andthe low-foaming resin cover material contain a thermoplastic resin whichis thermoplastic, together with a foaming agent. The resin covermaterial contains a thermoplastic resin.

The thermoplastic resin described above need not be particularlyspecified as it can include at least one of resins such as low-densitypolyethylene, high-density polyethylene, straight-chain low-densitypolyethylene, random polypropylene, homopolypropylene, blockpolypropylene, polyvinyl chloride, chlorinated polyvinyl chloride, ABSresin, polystyrene, polycarbonate, polyamide, polyvinylidene fluoride,polyphenylene sulfide, polysulfone, polyether ether ketone, orcopolymers thereof.

Using one, or a mixture of, olefin resins such as low-densitypolyethylene, high-density polyethylene, straight-chain low-densitypolyethylene, random polypropylene, homopolypropylene, or blockpolypropylene as the thermoplastic resin is preferred in the highfoaming resin core material, the low foaming resin cover material, andthe resin cover material because the plastic foam material would haveimproved resiliency. In particular, high-density polyethylene andhomopolypropylene are especially desirable

The thermoplastic resin used in the high foaming resin core material andthe low foaming resin cover material can be cross-linked. The methodused for cross-linking may include the same methods recited previouslywith regards to embodiment 3.

The silane-modified thermoplastic resins for use in melt-mulling aresimilar to the ones used in embodiment 3.

Similar to embodiment 3, silane-modified thermoplastic resin refers to athermoplastic resin modified with an unsaturated silane compound.

Generally, unsaturated silane compounds are expressed as

    RR'.sub.m SiY.sub.3-m

where:

R includes an alkenyl exemplified by vinyl, allyl, propenyl, orcyclohexenyl or an organic functional group such as a halogenated alkylexemplified by glycidyl, amino, methacryl, γ-chloroethyl, orγ-bromoethyl;

R' includes an aliphatic saturated aromatic hydrocarbon such as methyl,ethyl, propyl, or decyl;

Y includes a hydrolytic organic function group such as methoxy, ethoxy,formyloxy, propionoxy allyl amino; and

m is 0, 1, or 2.

In particular, an unsaturated silane compound represented by

    CH.sub.2 ═CHSi(OA).sub.3

is desirable because of its fast cross-linking reaction,

where

A is an aliphatic saturated hydrocarbon with 1 to 8 carbons, with 1 to 4carbons being preferred. Examples of the unsaturated compoundexemplified by CH₂ ═CHSi(OA)₃ include vinyl trimethoxy silane, vinyltriethoxy silane, and vinyl triacetoxy silane.

The method for making the silane-modified thermoplastic resin describedabove can be a conventional method, and is not particularly specified.For example, a silane-modified polyethylene can be made by performing areaction with polyethylene, an organic peroxide, and an unsaturatedsilane compound expressed as

    RR'SiY.sub.2,

where:

R is an olefin unsaturated group I hydrocarbon or a hydrocarbonoxy,

Y includes a hydrolytic organic group, and

R' is either a R group or a Y group as defined above.

In silane-modified thermoplastic resins having a silyl group, forexample, if Y is a methoxy, contact with water leads to hydrolysis, thuscreating a hydroxyl. The hydroxyl of different molecules react, creatingan Si-O-Si bond, thereby effectively cross-linking the silane-modifiedthermoplastic resins.

The method for mixing the silane-modified thermoplastic resin is similarto embodiment 3.

The water processing step including is also similar to the step utilizedin the third embodiment.

Similar to the third embodiment, water processing may be performed byexposure to steam under pressure and at a temperature higher than 100°C.

If the water or steam temperature during water processing is too low,the cross-linking reaction proceeds too slowly. If the temperature istoo high, the outer materials fuse to each other. Therefore, atemperature of from about 50° to about 130° C. is desirable, preferablyfrom about 90° to about 120° C.

If the duration of the water processing is too short, the cross-linkingreaction may not proceed to completion, and if the duration is too long,the outer materials may fuse to each other. Therefore, a duration timeof from 5 minutes to about 12 hours is preferred.

Excessive amounts of the silane-modified thermoplastic resin causesexcessive cross-linking which in turn yields a plastic foam materialwith a low expansion ratio. Alternatively, if the amount is too small,the foam cells burst, preventing uniform cells. Therefore, about 5 toabout 50 parts by weight of the silane-modified thermoplastic resin to100 parts by weight of thermoplastic resin is preferred, more preferablyfrom about 20 to about 35 parts by weight.

Similar to the previous embodiments, a cross-linking catalyst may beadded if necessary to effectively cross-link the various resins.Suitable examples of cross-linking catalysts have been described withregard to the previous embodiments.

It is preferable that the amount of the cross-linking catalyst per 100parts by weight of the thermoplastic resin be from about 0.001-10 partsby weight, and more preferably from about 0.01 to about 0.1 parts byweight.

The peroxide used in the cross-linking method is the same as previouslydescribed. The peroxide for use in the cross-linking gent is similar tothe one used in the previous embodiments. In particular, dicumylperoxide and tertian butyl cumyl peroxide are desirable, and dicumylperoxide is especially desirable.

For 100 parts by weight of the thermoplastic resin, it is desirable touse from about 0.5 to about 5 parts by weight of a peroxide. From about1 to about 3 parts by weight is especially desirable.

The method for cross-linking the thermoplastic resins using radiation issimilar to embodiment 3, including the specified dosages. From about 1to about 20 Mrad of radiation dosage is preferred, with about 3 to about10 Mrad being optimal.

The method for irradiating the thermoplastic resins may include the useof two electron beam generating devices between which a thermoplasticresin is passed, in order to irradiate the thermoplastic resins.

The "high foaming" and "low foaming" referred to in the high foamingresin core material and the low foaming resin cover material are labelsreferring to the relative expansion ratios of the two. Of the two foamresins, the one which produces the plastic foam material with the higherexpansion ratio is called the high foaming resin, while the other iscalled the low foaming resin.

The foaming agent used in the high foaming resin core material and thelow foaming resin cover material is not particularly specified as longas it has a higher decomposition point than the melting point of thethermoplastic resin being used. Suitable examples of foaming agent foruse in conjunction with the high foaming core material and the lowfoaming resin cover material include inorganic thermodecomposing foamingagents such as sodium bicarbonate, ammonium carbonate, ammoniumbicarbonate, azide compounds, sodium bicarbonate, azodicarbonamide,azobisisobutylonitryl, N,N'-dinitrosopentamethylene tetramine,P,P'-dinitrosopentamethylene tetramine, P,P'-oxybisbenzenesulfonyl,hydrazide, barium azodicarbonate, trihydradinotriazine. In particular,azodicarbonamide is desirable.

The expansion ratio of the cover material and the core material can beadjusted through selection of the type of thermoplastic resin used inthe foaming resin, the degree of polymerization, crystallization,presence of cross-linking, density, type of thermodecomposing foamingagent and amount of foam added.

The expansion ratio is adjusted by controlling the amount of foamingagent added. In such cases, if the amount of foaming agent added is toohigh, cells break, preventing uniform cell formation and decreasing thecompression strength of the plastic foam material. If the amount addedis too low, foaming does not occur.

Accordingly, for 100 parts by weight of thermoplastic resins, it ispreferable that the amount of the foaming agent be from 1 to about 25parts by weight.

Adjustments can be made by finding the gas yield corresponding to thedesired expansion ratio, and adding an amount of foaming agent capableof generating this gas yield.

The method for making the shaped foamable piece is not particularlyspecified. In one method, the thermoplastic resin for the high-foamingresin core material and a foaming agent or the like are fed to atwin-axis extruder. The resulting resin is melt-mulled at a temperaturelower than the decomposition point of the thermodecomposing foamingagent.

The thermoplastic resin for the low-foaming resin cover material, afoaming agent and the like, or the thermoplastic resin for thelow-foaming resin cover material are fed into a different twin-axisextruder and then co-extruded at a temperature lower than thedecomposition point of the thermodecomposing foaming agent.

Co-extrusion is performed to extrude a strand-shaped body, which is thencut to prescribed dimensions.

In yet another method, the thermoplastic resin for the high foamingresin core material and a foaming agent are fed to a twin-axis extruder.The resin is extruded at a temperature lower than the decompositionpoint of the foaming agent. A strand-shaped body is extruded and cut.This is then immersed in an organic solvent in which is dissolved eithera low foaming resin or a thermoplastic resin.

If the method involving immersion in an organic solvent is used, theorganic solvent is not particularly specified as long as it can dissolvethe thermoplastic resin used in the cover material, and thethermodecomposing foaming agent. Examples include aromatic hydrocarbonssuch as toluene and xylene, halogenated hydrocarbons such as methylenechloride and dichloromethane, and ketone compounds such as acetone andmethylethyl ketone.

Specifically, it is desirable to use methylene chloride if thethermoplastic resin for the cover material is polystyrene or polyvinylchloride, and it is desirable to use methylethyl ketone if polyvinylchloride is to be used.

The shape of the foamable piece is not important. The shape can includea circular column or a polygonal column such as a square column or atriangular column. A circular column shape should have a symmetricalcross-section in the plastic foam material, so that there is nofluctuation in foaming. This permits the cover material or thethermoplastic resin body to have a neat lattice shape or honeycombshape, thus providing improved rigidity in the compression strength inthe final plastic foam material.

When a circular column shape is used as the foamable piece, its averagecross-sectional diameter should be from about 3 to about 20 mm, with adiameter of from 5 to about 10 mm being preferred.

When the diameter of the circular column shape exceeds 20 mm, the ratioof width to height is thought to be considerably high, whichsubstantially weakens the plastic foam material. Alternatively, if thediameter of the foamable circular shape falls below 3 mm, the plasticfoam material is very heavy.

If the low-foaming resin cover material is thicker than 3 mm, theplastic foam material is increasingly heavy. If the low-foaming resincover material is thinner than 0.05 mm, the compression strength of theplastic foam material is considerable weakened.

For similar reasons, it is desirable that the thickness of the resincover material be from about 0.05 to about 5.0 mm, and more preferablethat the thickness be from about 0.5 to about 3.0 min.

The plurality of shaped foam pieces used in the present embodiment needsto have the same height. In the present embodiment however, they do notneed to have the exact same heights as long as their heights are roughlythe same.

The thickness regulating bodies used in the present embodiment aredetermined by a heating method but are not particularly specified. Forexample, if heating is done by circulating oil, metal can be used. Ifheating is done through hot air, mesh can be used.

The thickness regulating bodies can be stationary or they can be movingat a fixed speed.

However, if the thickness regulating bodies are driven at a fixed speed,the pair of thickness regulating bodies need to move in the samedirection at the same speed.

The method of feeding the shaped foam pieces to the first thicknessregulating body is not particularly specified as long as the bottomsurface of the shaped foam pieces touch the first thickness regulatingbody. For example, a rotating roller having holes for the shaped foampieces arranged perpendicular to the circumference can be used so thatshaped foam pieces are supplied to the upper part, and the shaped foampieces fall straight down from the lower part.

It is desirable that the shaped foamable pieces be arranged in a roughlyuniform manner so that the expansion ratio of the plastic foam materialdoes not exhibit local variations.

When the shaped foam pieces are placed on the first thickness regulatingbody so that their bottom surfaces touch the regulating body andadjacent cover materials fuse together during foaming, it is desirableto arrange the foam pieces so that they have fixed vertical andhorizontal intervals. This permits the cross section of thethermoplastic resin or the cover material to form an orderly lattice,thus increasing compression strength in the final plastic foam material.

Also, by arranging the shaped foam pieces so that the center of thebottom surface is positioned at the center and vertices of hexagonsmaking up an imaginary honeycomb arranged on the first thicknessregulating body, the cross section of the cover material or thethermoplastic resin is an orderly honeycomb shape. This provides furtherimprovements in the compression strength of the final plastic foammaterial.

The center of the bottom surface of a shaped foamable piece referred tohere refers not to the precise center, but to the central region of thebottom surface.

The length of a side of the hexagon making up the honeycomb shape isdetermined by factors such as the expansion ratio of the cover and corematerials, the thickness of the thermoplastic resin and the desiredplastic foam material compression strength. If the sides are too long,the compression strength of the plastic foam material decreases, whileif they are too short, the expansion ratio of the plastic foam materialdecreases. Therefore a length of 5-100 mm is desirable, with a range of10-50 mm being especially desirable.

Furthermore, it is preferred that the hexagons which make up theimaginary honeycomb shape on the first thickness regulating body shouldbe equilateral. Further, it is preferred that these hexagons fulfill theconditions in the equation:

    R.sup.2 ≦L.sup.2 ≦TS/√3,

where:

L is the length of one side of the equilateral hexagon,

T is the expansion ratio of the core material,

S is the area of the bottom surface of the shaped foam piece, and

R is the maximum length of the bottom surface of the shaped foam piece.(The maximum length of the bottom surface of the shaped foam piecerefers to the length of the longest line on the bottom surfaceconnecting the two points on the outer perimeter of the bottom surface).

When using circular column-shaped foamable pieces, if the hexagonsmaking up the imaginary honeycomb on the first thickness regulating bodyare equilateral, it is preferred that the high foaming resin corematerial column body to fulfill the conditions of the equation:

    4.sup.r.sup.2 ≦L'.sup.2 ≦T'rπ/√3,

where:

L' is the length of one side of an equilateral hexagon,

T' is the expansion ratio of the core material, and

r is the radius of the bottom surface of the high foaming resin corematerial column, which is covered by the low foaming resin covermaterial.

When shaped foamable pieces with a high-foaming resin core materialmaterials covered entirely by low foaming resin cover materials or resincover materials are used, the method of sending them to the firstthickness regulating body is not particularly specified as long as theydo not overlap. One possible method is to scatter the shaped foam piecesarbitrarily onto the first thickness regulating body, and then vibratethe first thickness regulating body so that overlaps are eliminated andthe shaped foam pieces are distributed evenly.

The use of a reinforcing sheet between the thickness regulating body andthe shaped foamable pieces improves the compression strength of theplastic foam material and is desirable. The sheet can comprise materialsuch as glass paper, chopped-strand mats, thermoplastic resin sheets,thermosetting resin sheets, metal sheets, or the like.

If the glass paper or the glass fibers used therein are too heavy, theplastic foam material is too heavy. If they are too light, the strengthof the plastic foam material is inadequate. Therefore a weight of fromabout 10 to about 100 g/m² is desirable, with 20 to about 50 g/m² beingespecially desirable.

The thermoplastic resin used in the thermoplastic resin sheet describedabove is not particularly specified and can be made from polyethylene,polypropylene, polyethylene terephthalate, or the like. It is desirableto use the same type of polyolefin resin as is used in the plastic foammaterial so that the sheet and the plastic foam material can adhere wellto each other.

The thermosetting resin used in the thermosetting resin sheet describedabove is similar to the ones used in embodiment 3.

The metal used in the metal sheet described above is similar to the oneused in embodiment 3.

Similar to the previous embodiment, if the sheet described above is toothick, the resulting plastic foam material is too heavy. Alternatively,if the sheet is very thin, the plastic foam material is not reinforcedadequately. Accordingly, a sheet measuring in thickness from about 0.05to about 1 mm is preferred, with a thickness of from about 0.1 to about0.5 mm being optimal.

The details of the method involving supporting the shaped foam piecesbetween the thickness regulating bodies and applying heat while a fixeddistance is maintained between the thickness regulating bodies are notparticularly specified as long as the temperature goes above thedecomposition point of the thermodecomposing foaming agent forming thehigh-foaming resin core material and the low-foaming resin covermaterial.

For example, heat can be applied using an electric heater, afar-infrared radiation heater or a heating device circulating a heatingmedium such as oil or air.

To reiterate, the plastic foam material of the present embodimentcomprises a cover material and a core material, or a thermoplastic resinbody and a core material. The cover material and the thermoplastic resinbody have a cross-section shaped in the form of a lattice or ahoneycomb.

The cover material and the thermoplastic resin are, in relative terms,low foaming or non-foaming, providing superior compression strength. Onthe other hand, the core material is relatively high foaming, andalthough compressibility decreases, it is very light. Therefore, in thepresent embodiment, the cover material and the thermoplastic resin bodymake up for the shortcomings of the core material, while conversely, thecore material makes up for the shortcomings of the cover material or thethermoplastic resin body. This provides a product that is light and hassuperior compression strength.

The expansion ratio of the core material generally ranges from about 10to about 50, a range in which lightness is not greatly compromised.Since the expansion ratio of the cover material is lower than that ofthe core material by a ratio of 5 or more, compression strength is good.

Furthermore, since the cover material and the thermoplastic resin have across-section shape of a lattice or honeycomb-shape, the compressionstrength is further improved.

In the plastic foam material of the present embodiment, the covermaterial and the thermoplastic resin body form a lattice, providingsuperior properties such as compression strength. This is the result ofthe method for making foam bodies of the present embodiment, in whichshaped foam pieces, comprising a core material and a cover materialcovering at least the side surfaces of the core material, are arrangedso that their bottom surfaces are on a first thickness regulating bodyand so that they form a lattice pattern in which adjacent covermaterials fuses together.

A second thickness regulating body is arranged above, and foaming takesplace while a fixed distance is maintained between the thicknessregulating bodies. This makes foaming of the shaped foam piece possibleonly in the direction parallel to the surfaces of the thicknessregulating bodies, and the arrangement is such that the cover materialsfuse together.

Furthermore the final foam bodies have improved properties such ascompression strength because the cross-section of the cover material andthe thermoplastic resin forms a honeycomb shape. This is the result ofarranging the shaped foam pieces on the first thickness regulating bodyso that the center of the bottom surfaces are positioned at the centerand the vertices of the hexagons making up an imaginary honeycomb-shapearranged on the first thickness regulating body, and so that adjacentcover materials fuse together when foaming takes place. The secondthickness regulating body is then arranged from the top, and heat isapplied so that foaming can take place while a fixed interval ismaintained between the thickness regulating bodies.

Also, during this process, if the arrangement fulfills the conditions ofthe equation:

    R.sup.2 ≦L.sup.2 ≦TS/√3,

the cross-section of the thermoplastic resin body and the core materialcan form an orderly honeycomb shape, thus improving the properties ofthe plastic foam material such as the compression strength.

If the shaped foam pieces are shaped as circular columns and arearranged to fulfill the conditions of the equation:

    4r.sup.2 ≦L'.sup.2 ≦T'rπ/√3,

foaming can take place evenly because of the symmetry of the shaped foampieces. Thus, an even more orderly honeycomb shape is provided for thecross section of the cover material and the thermoplastic resin body,and the properties of the plastic foam material such as compressionstrength are improved.

If shaped foam pieces in which the entire surface of the core materialis covered by the cover material, there is no need to arrange them onthe first thickness regulating body so that their bottom surfaces touchthe thickness regulating body, This simplifies the implementation of theembodiment.

The following is a description of the aforementioned embodiment of thepresent invention, which should be read while referring to theaccompanying drawings.

Referring to FIG. 13, there is shown a production device used in theaforementioned embodiment. "Front" or "forward" refers to the rightwarddirection in the drawing.

Referring to FIG. 13, there are shown two continuous drive belts 51a and51b. The distance between continuous drive belts 51a, 51b is 25 mm.Lower continuous drive belt 51b extends further toward the rear thanupper continuous drive belt 51a. A preheating device 53, a heatingdevice 54, two thickness regulating plates 55a and 55b, and a coolingdevice 56 are arranged in that order, starting from the back. Thicknessregulating plates 55a, 55b are separated by a distance of 27 min.

EXAMPLE 38

High-density polyethylene exemplified by Mitsubishi Petrachemicalsproduct name EY 40H, polypropylene exemplified by MitsubishiPetrachemicals product name PY230, silane graft polypropyleneexemplified by Mitsubishi Petrachemicals product name XPM 800H, andazodicarbonamide are contained in the amounts shown in Table 6 in thecomposition forming the high-foaming resin core material. Thiscomposition is then fed to a 50 mm diameter single axis extruder,melt-mulled at 180° C., and extruded as a circular strand having a 5 mmdiameter circular cross-section. High-density polyethylene exemplifiedby Mitsubishi Petrochemicals product name EY40H, polypropyleneexemplified by Mitsubishi Petrochemicals product name PY230,silane-modified thermoplastic polypropylene exemplified by MitsubishiPetrochemicals product name XPM800H, and azodicarbonamide are containedin the amounts shown in Table 6 in the composition forming the lowfoaming resin core material. This composition is then fed to a 25 mmdiameter single-axis extruder, melt-mulled at 180° C., anddouble-extruded with a cross-head die so that it covers the stranddescribed above. A strand with a circular cross-section is extruded,cooled, cut to 25 mm lengths, immersed in water for one hour at 98° C.,and dried. This produces a shaped foam piece 52 which is 25 mm high andhas a low foaming resin cover material 1.0 mm thick and a high-foamingresin core material 5 mm in diameter.

The resulting shaped foam pieces 52 are placed on lower continuous drivebelt 51b in the quantity 1325 g/m² in a lattice arrangement so that thebottom surfaces touch the drive belt and adjacent cover materials fusetogether. Shaped foam pieces 52, arranged between continuous drive belts51a,51b are heated to 200° C. by preheating device 53. Thicknessregulating bodies 55a, 55b maintain a fixed distance between continuousdrive belts 51a, 51b while heating device 54 applies 220° C., foamingthe foam pieces 52. Cooling device 56 cools to 30° C., resulting in aplastic foam material 57 in which the cross-section of the core materialforms a lattice shape.

Table 6 shows the results of measuring the following factors accordingto the methods described below: the expansion ratios of the corematerial and the cover material, the bending strength of plastic foammaterial 57, the 25% compression strength, compression setting, and heatconductivity.

                                      TABLE 5                                     __________________________________________________________________________                      Comparative        Comparative                              Example 34  Example 35                                                                          Example 14                                                                           Example 36                                                                          Example 37                                                                          Example 15                               __________________________________________________________________________    d.sub.2                                                                             30    30    30     30    30    30                                       d.sub.1                                                                             31.07 31.16 31.2   30.93 31.92 31.2                                     d.sub.3                                                                             32.07 32.16 32.20  31.93 32.02 32.20                                    TI    15    15    15     15    15    15                                       TO    1     1     1      3     3     3                                        Length of                                                                           9.2   2.6   0      9.3   2.9   0                                        center hole                                                                         2.9   0                                                                 (mm)                                                                          Amount of                                                                           18000 250   0      18000 300   0                                        water                                                                         penetrating                                                                   (cc)                                                                          __________________________________________________________________________     d.sub.2 is inner diameter of inner layer in mm                                d.sub.1 is inner diameter of outer layer in mm                                d.sub.3 is outer diameter of outer layer in mm                                TI is expansion ratio of inner layer                                          TO is expansion ratio of outer layer                                     

                                      TABLE 6                                     __________________________________________________________________________                       Example                                                                       38   39  40   41  42   43  44   45  46  47                 __________________________________________________________________________    High foaming resin core material layer                                        High-density polyethylene                                                                        50   50  50   50  50   50  50   50  50  50                 Polypropylene      50   50  50   50  50   50  50   50  50  50                 Silane graft polyethylene                                                                        20   20  20   20  20   20  20   20  20  20                 Azodicarbonamide   8    8   8    8   8    8   8    8   8   8                  Low foaming resin cover material layer                                        High-density polyethylene                                                                        50   50  --   --  50   50  50   50  50  50                 Polypropylene      50   50  --   --  50   50  50   50  50  50                 Silane graft polyethylene                                                                        20   20  --   --  20   20  20   20  20  20                 Azodicarbonamide   2    2   2    2   2    2   2    2   2   2                  Polystyrene        --   --  100  100                                          Shape of shaped foam pieces                                                                      circ.                                                                              circ.                                                                             circ.                                                                              circ.                                                                             sq.  sq. sq.  circ.                                                                             circ.                                                                             sq.                Distribution (g/m.sup.2)                                                                         1325 1325                                                                              1325 1325                                                                              1325 1280                                                                              1325 1325                                                                              1325                                                                              1325               Glass paper (Yes/No)                                                                             N    Y   N    N   N    N   N    N   N   Y                  Area of core material covered by                                                                 sides                                                                              sides                                                                             all  all sides                                                                              sides                                                                             sides                                                                              sides                                                                             sides                                                                             sides              cover layer                                                                   Arrangement of shaped foam pieces                                                                n1   n1  n1   n2  n3   n3  n4   n3  n5  n3                 Expansion factor of core layer                                                                   20   20  20   20  20   20  20   20  20  20                 Expansion factor of cover layer                                                                  5    5   5    5   5    12.5                                                                              5    5   5   5                  Bending strength (Kg/cm.sup.2)                                                                   8.5  18.7                                                                              9.2  9.0 8.7  8.2 9.5  8.9 9.8 20.7               25% compression strength (kg/cm.sup.2)                                                           2.30 2.34                                                                              5.00 4.87                                                                              4.80 2.38                                                                              5.02 4.98                                                                              5.38                                                                              4.86               Compression setting (%)                                                                          7.1  7.3 8.2  8.1 7.3  7.1 7.5  7.4 7.7 7.4                Heat conductivity (kcal/mhr °C.)                                                          0.03 0.03                                                                              0.03 0.03                                                                              0.03 0.025                                                                             0.03 0.03                                                                              0.03                                                                              0.03               __________________________________________________________________________     Notes n1-n5 are explained at the bottom of Table 8.                      

The expansion ratio is measured according to JIS K6767. The bendingstrength is measured according to JIS A9511. The 25% compressionstrength is measured according to JIS K6767. The compression setting ismeasured according to JIS K6767. The heat conductivity is measuredaccording to JIS A1413.

EXAMPLE 39

Glass paper exemplified by Oribest Corp. product name FEO-025 isarranged on lower continuous drive belt 51b. Shaped foam pieces 52 asobtained in Example 38 are arranged on the glass paper at 1325 g/m² sothat the bottom surface touches the glass paper and so that adjacentcover materials fuse together. Glass paper exemplified by Oribest Corp.product name FEO-025 is also arranged between the shaped foam pieces andupper continuous drive belt 51a. Otherwise a plastic foam material 57with the core material having a lattice-shaped cross-section is obtainedin the same way as in Example 38.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 40

A composition indicated in Table 6 for a high foaming resin corematerial, as in Example 38, is fed to a 50 mm diameter single-axisextruder, melt-mulled at 180° C., and extruded as a strand having acircular cross section 5 mm in diameter. The resulting strand is cut inlengths of 25 mm, immersed in water for an hour at 98° C., and then leftto dry. This produces a preliminary shaped foam piece 25 mm in heightand having a 5 mm diameter.

The preliminary shaped foam piece is immersed for 10 minutes in anorganic solvent solution formed by a polystyrene exemplified by AsahiKasei Kogyo Corp. Ltd. product name GP Staroyn 691 and azodicarbonamide,in the amounts shown in Table 6, dissolved in 30 parts by weight ofmethylene chloride. This is then dried at 25° C. The process is repeatednine times to produce shaped foam pieces 52 having a low foaming resincover material 1.00 mm thick.

Resulting foam pieces 52 are processed in the same way as in Example 38to produce a plastic foam material 57 with the core materialcross-section forming a lattice shape.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 41

A plastic foam material 57 with the core material cross-section forminga lattice shape is produced in the same manner as in Example 40 exceptthat shaped foam pieces 52 arranged on lower continuous drive belt 51bdo not necessarily have their lower surfaces touching continuous drivebelt 51b as long as the foam pieces do not overlap.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 42

A high-foaming resin core material composition identical to the one inExample 38, as shown in Table 6, is fed to a 50 mm diameter single-axisextruder, melt-mulled at 180° C., and extruded as a strand with arectangular cross-section having 4 mm sides. A low-foaming resin covermaterial composition identical to the one in Example 38, as shown inTable 6, is fed to a 25 mm diameter single-axis extruder, melt-mulled at180° C., co-extruded with a cross head die, cooled, cut into 25 mmlengths, immersed in water at 98° C. for an hour and left out to dry.The result is shaped foam pieces 52 with low-foaming resin covermaterials 1.0 mm thick, with high-foaming resin core material materialshaving square 4 mm×4 mm cross-sections, with a height of 25 mm.

Resulting shaped foam pieces 52 are arranged at 1325 g/m² on lowercontinuous drive belt 51b so that their bottom surfaces touch the drivebelt and so that the center of their bottom surfaces are positioned atthe centers and vertices of hexagons that form an imaginary honeycombarranged on lower continuous drive belt 51b. Shaped foam pieces 52,arranged between continuous drive belts 51a,51b, are heated bypreheating device 53 at 200° C. and then heated by heating device 54 at220° C. while thickness regulating bodies 55a, 55b maintain a fixeddistance between continuous drive belts 51a, 51b to produce foaming.After foaming, cooling device 56 cools the product to 30° C. The resultis plastic foam material 57 whose cover materials have ahoneycomb-shaped cross-section.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 43

Shaped foam pieces 52 are produced according to the same method used inExample 38 except that the amount of azodicarbonamide added to producethe low-foaming resin cover material is 5 parts by weight.

Using these shaped foam pieces 52, plastic foam material 57 with a covermaterial having a honeycomb-shaped cross-section is produced in the samemanner as in example 5 except that a distribution of 1280 g/cm² is used.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 44

Using shaped foam pieces 52 as produced in Example 42, plastic foammaterial 57 with a cover material having a honeycomb-shapedcross-section is produced in the same manner as in Example 42 exceptthat shaped foam pieces 52 are arranged on lower continuous drive belt51b so that each shaped foam piece 52 is positioned on a vertex orcenter of an equilateral hexagon having 13 mm sides making up animaginary honeycomb arranged on lower belt drive 51b. The length L ofthe sides of the equilateral hexagons making up the honeycomb is 13 mm,expansion ratio T of the cover material is 20, the area S of the bottomsurface of the shaped foam pieces is 17.64 mm², and the maximum length Rof the bottom surface of the shaped foam pieces is 5.93 mm.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 45

Using shaped foam pieces 52 as produced in Example 38, plastic foammaterial 57 with the cover material having a honeycomb-shapedcross-section is produced in the same manner as in example 42.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 46

Using shaped foam pieces 52 as produced in Example 38, plastic foammaterial 57 with the cover material having a honeycomb-shapedcross-section is produced in the same manner as in Example 44. Thelength L' of the sides of the equilateral hexagons making up thehoneycomb is 15 mm, the expansion ratio T' of the cover material is 20,and the radius r of the bottom surface of the shaped foam pieces is 2.6mm².

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 47

Plastic foam material 57 with the cover material having ahoneycomb-shaped cross-section is produced in the same manner as inExample 42 except that glass paper exemplified by Oribest Corp. productname FEO-025 is arranged on lower continuous drive belt 51b. Shaped foampieces 52 as obtained in Example 42 are arranged on the glass paper at1325 g/m² so that the bottom surface touches the glass paper and so thatadjacent cover materials fuse together. Glass paper exemplified byOribest Corp. product name FEO-025 is also arranged between the shapedfoam pieces 52 and upper continuous drive belt 51a.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 6.

EXAMPLE 48

High-density polyethylene exemplified by Mitsubishi Petrochemicalsproduct name EY 40H, polypropylene exemplified by MitsubishiPetrochemicals product name PY230, silane-modified thermoplasticpolypropylene exemplified by Mitsubishi Petrochemicals product name XPM800H, and azodicarbonamide are contained, in the amounts shown in Table8, in the composition forming the high-foaming resin core material. Thiscomposition is then fed to a 50 mm diameter single axis extruder,melt-mulled at 180° C., and extruded as a circular strand having a 5 mmdiameter circular cross-section. Polypropylene exemplified by MitsubishiPetrochemicals product name PY230, which makes up the resin corematerial, is fed to a 25 mm diameter single-as extruder, melt-mulled at180° C., and co-extruded with a crosshead die so that it covers thestrand described above. A strand with a circular cross-section isextruded, cooled, cut to 25 mm lengths, immersed in water for one hourat 98° C. and dried. This produces a shaped foam piece 52 which is 25 mmhigh and has a low-foaming resin cover material 1.0 mm thick and ahigh-foaming resin core material 5 mm in diameter.

                  TABLE 7                                                         ______________________________________                                                   Comparative Examples                                                          16    17      18      19    20                                     ______________________________________                                        High foaming resin                                                            core material layer                                                           High-density 50      50      50    50    50                                   polyethylene                                                                  Polypropylene                                                                              50      50      50    50    50                                   Silane graft 20      20      20    20    20                                   polyethylene                                                                  Azodicarbonamide                                                                           8       3       8     8     3                                    Low foaming resin                                                             cover material layer                                                          High-density --      50      50    --    50                                   polyethylene                                                                  Polypropylene                                                                              --      50      50    --    50                                   Silane graft --      20      20    --    20                                   polyethylene                                                                  Azodicarbonamide                                                                           --      2       7     --    2                                    Shape of shaped foam                                                                       circ.   circ.   circ. sq.   circ.                                pieces                                                                        Distribution (g/m.sup.2)                                                                   1250    3420    1260  1250  3420                                 Glass paper (Yes/No)                                                                       N       N       N     N     N                                    Area of core material                                                                      sides   sides   sides sides sides                                covered by cover                                                              layer                                                                         Arrangement of                                                                             n1      n1      n1    n3    n3                                   shaped foam pieces                                                            Expansion ratio of                                                                         20      7.5     20    20    7.5                                  core layer                                                                    Expansion ratio of                                                                         --      5       17.5  --    5                                    cover layer                                                                   Bending strength (kg/                                                                      6.5     12.8    7.2   6.5   12.8                                 cm.sup.2)                                                                     25% compression                                                                            1.23    9.07    1.39  1.23  9.39                                 strength (kg/cm.sup.2)                                                        Compression setting                                                                        6.1     14.5    6.5   6.1   15.8                                 (%)                                                                           Heat conductivity                                                                          0.02    0.04    0.025 0.02  0.04                                 (kcal/mhr °C.)                                                         ______________________________________                                         n1: Bottom surfaces touch the first thickness regulating body.                n2: Mounted at centers and vertices of hexagons forming an imaginary          honeycomb on the first thickness regulating body.                        

                                      TABLE 8                                     __________________________________________________________________________                       Example                                                                       48  49  50  51  52  53  54  55  56                         __________________________________________________________________________    High foaming resin core material layer                                        High-density polyethylene                                                                        50  50  50  50  50  50  50  50  50                         Polypropylene      50  50  50  50  50  50  50  50  50                         Silane graft polyethylene                                                                        20  20  20  20  20  20  20  20  20                         Azodicarbonamide   8   8   8   8   8   8   8   8   8                          Shape of shaped foam pieces                                                                      circ.                                                                             circ.                                                                             circ.                                                                             circ.                                                                             sq. sq. circ.                                                                             circ.                                                                             sq.                        Distribution (g/m.sup.2)                                                                         1325                                                                              1325                                                                              1325                                                                              1325                                                                              1325                                                                              1325                                                                              1325                                                                              1325                                                                              1325                       Glass paper (Yes/No)                                                                             N   Y   N   N   N   N   N   N   Y                          Area of core material covered by                                                                 sides                                                                             sides                                                                             all all sides                                                                             sides                                                                             sides                                                                             sides                                                                             sides                      cover layer                                                                   Arrangement of shaped foam pieces                                                                n1  n1  n1  n2  n3  n4  n3  n5  n3                         Expansion ratio of core layer                                                                    20  20  20  20  20  20  20  20  20                         Bending strength (Kg/cm.sup.2)                                                                   9.3 20.9                                                                              10.5                                                                              10.2                                                                              9.5 10.3                                                                              9.8 10.0                                                                              23.8                       25% compression strength (kg/cm.sup.2)                                                           2.81                                                                              2.82                                                                              6.22                                                                              6.08                                                                              5.72                                                                              6.21                                                                              6.20                                                                              6.51                                                                              6.09                       Compression setting (%)                                                                          7.5 7.6 10.1                                                                              10.1                                                                              7.6 7.9 7.7 10.2                                                                              7.7                        Heat conductivity (kcal/mhr °C.)                                                          0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                       __________________________________________________________________________     n1: Bottom surfaces touch first thickness regulating body.                    n2: No overlapping of shaped foam pieces occurs.                              n3: Mounted at centers and vertices of hexagons forming an imaginary          honeycomb on the first thickness regulating body.                             n4: Same as n3, except hexagons are equilateral and R.sup.2 ≦          L.sup.2 ≦ TS/√3.                                                n5: Same as n3, except hexagons are equilateral and 4r.sup.2 ≦         L'.sup.2 ≦ T'rπ/√3.                                     

Using these shaped foam pieces 52, plastic foam material 57 with athermoplastic resin body having a lattice-shaped cross-section isproduced in the same manner as in Example 38.

The expansion ratios of the cover material and the core material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, 25 percent compression strength, compression setting andheat conductivity were measured using the same methods as in Example 38,and the results are shown in Table 8.

EXAMPLE 49

A plastic foam material with a thermoplastic resin having alattice-shaped cross-section is produced in the same manner as inExample 39 except that shaped foam pieces 52 obtained from Example 48are used.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 50

The high-foaming resin core material composition from example 48, asshown in Table 8, is fed to a 50 mm diameter single-axis extruder,melt-mulled at 180° C., extruded as a strand having a circularcross-section 5 mm in diameter, cut in 25 mm lengths, immersed in waterfor one hour at 98° C. and left to dry. This produces preliminary shapedfoam pieces 5 mm in diameter and 25 mm high.

The resulting preliminary shaped foam pieces are immersed for 10 minutesin an organic solvent solution, in which 100 parts by weight ofpolystyrene exemplified by Asahi Kasei Kogyo Corp. Ltd. product name GPStaroyn 691 is dissolved in 30 parts by weight of methylene chloride.This is then dried at 25° C. The process is repeated nine times toproduce shaped foam pieces 52 with a 1.0 mm thick resin cover material.

Using resulting shaped foam pieces 52, plastic foam material 57 with athermoplastic resin body having a lattice-shaped cross-section isproduced in the same manner as in Example 38.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 51

Plastic foam material 57 with a thermoplastic resin body having alattice-shaped cross-section is produced in the same manner as inexample 4 except for the use of shaped foam pieces 52 obtained fromExample 50.

The expansion ratios of the core material making up final plastic foammaterial 57, the bending strength of plastic foam material 57, the 25%compression strength, the compression setting, and the heat conductivityare measured using the same methods as in Example 38, and the resultsare shown in Table 8.

EXAMPLE 52

The high-foaming resin core material composition from Example 48, asshown in Table 8, is fed to a 50 mm diameter single-axis extruder,melt-mulled at 180° C., extruded as a strand having a squarecross-section with 4 mm sides, cut in 25 mm lengths, immersed in waterfor one hour at 98° C. and left to dry. This produces shaped foam pieceshaving 1.0 mm thick low-foaming resin cover materials, high-foamingresin core material materials with 4 mm×4 mm square cross sections, andheights of 25 mm.

Plastic foam material 57 with a thermoplastic resin body having ahoneycomb-shaped cross-section is produced in the same manner as inExample 42 except that shaped foam pieces 52 described above are used.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 53

Plastic foam material 57 with a thermoplastic resin body having ahoneycomb-shaped cross-section is produced in the same way as describedin Example 44 except for the use of shaped foam pieces 52 obtained fromExample 52.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 54

Plastic foam material 57 with a thermoplastic resin body having ahoneycomb-shaped cross-section is produced in the same way as describedin Example 45 except for the use of shaped foam pieces 52 obtained fromExample 48.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 55

Plastic foam material 57 with a thermoplastic resin body having ahoneycomb-shaped cross-section is produced in the same way as describedin Example 56 except for the use of shaped foam pieces 52 obtained fromExample 48.

The expansion ratios of the core material and the cover material makingup final plastic foam material 57, the bending strength of plastic foammaterial 57, the 25% compression strength, the compression setting, andthe heat conductivity are measured using the same methods as in Example38, and the results are shown in Table 8.

EXAMPLE 56

Plastic foam material 57 with a thermoplastic resin body having ahoneycomb-shaped cross-section is produced in the same manner as inExample 52 except that glass paper exemplified by Oribest Corp. productname FEO-025 is arranged on lower continuous drive belt 51b. Shaped foampieces 52 as obtained in example 15 are arranged as in example 15. Glasspaper exemplified by Offbest Corp. product name FEO-025 is also arrangedbetween the shaped foam pieces 52 and upper continuous drive belt 51a.

The expansion ratios of the core material making up final plastic foammaterial 57, the bending strength of plastic foam material 57, the 25%compression strength, the compression setting, and the heat conductivityare measured using the same methods as in Example 38, and the resultsare shown in Table 8.

COMPARATIVE EXAMPLE 16

High-density polyethylene exemplified by Mitsubishi Petrochemicalsproduct name 40H, polypropylene exemplified by Mitsubishi Petrochemicalsproduct name PY230, silane-modified thermoplastic polypropyleneexemplified by Mitsubishi Petrochemicals product name XPM 800H, andazodicarbonamide are fed to a 50 mm diameter single-axis extruder in theamounts shown in Table 7, melt-mulled at 180° C., extruded as a strandhaving a circular cross-section 5.2 mm in diameter, cut to 25 mmlengths. The results are preliminary shaped foam pieces 5.2 mm indiameter and 25 mm high.

Using these preliminary shaped foam pieces, a plastic foam material isobtained in the same manner as in Example 38 except that thedistribution was set to 1250 g/m².

The expansion ratio of the core material making up the plastic foammaterial, the bending strength of the plastic foam material, the 25%compression strength, the compression setting, and the heat conductivityare measured using the same methods as in Example 38, and the resultsare shown in Table 7.

COMPARATIVE EXAMPLE 17

Shaped foam pieces were obtained in the same manner as in Example 38except that for 50 parts by weight of high-density polyethylene, 3 partsby weight of azodicarbonamide were added to form the high-foaming resincore material.

A plastic foam material was obtained in the same manner as in Example 38except that the distribution of the shaped foam pieces was set to 3420g/m².

The expansion ratios of the core material and the cover material makingup the plastic foam material, the bending strength of the plastic foammaterial, the 25% compression strength, the compression setting, and theheat conductivity are measured using the same methods as in Example 38,and the results are shown in Table 7.

COMPARATIVE EXAMPLE 18

Shaped foam pieces are obtained in the same manner as in Example 38except that for 50 parts by weight of high-density polyethylene, 7 partsby weight of azodicarbonamide are added to form the low-foaming resincover material.

A plastic foam material is obtained in the same manner as in Example 38except that the shaped foam pieces are distributed at 1260 g/m².

The expansion ratios of the core material and the cover material makingup the plastic foam material, the bending strength of the plastic foammaterial, the 25% compression strength, the compression setting, and theheat conductivity are measured using the same methods as in Example 38,and the results are shown in Table 7.

COMPARATIVE EXAMPLE 19

High-density polyethylene exemplified by Mitsubishi Petrochemicalsproduct name 40H, polypropylene exemplified by Mitsubishi Petrochemicalsproduct name PY230, silane-modified thermoplastic polypropyleneexemplified by Mitsubishi Petrochemicals product name XPM 800H, andazodicarbonamide are fed to a 50 mm diameter single-axis extruder in theamounts shown in Table 7, melt-mulled at 180° C., extruded as a strandhaving a 4.2 mm×4.2 mm square cross-section, and cut to 25 mm lengths.The results are preliminary shaped foam pieces 25 mm high.

Using these preliminary shaped foam pieces, a plastic foam material isobtained in the same manner as in example 5 except that the distributionwas set to 1250 g/m².

The expansion ratio of the core material making up the plastic foammaterial, the bending strength of the plastic foam material, the 25%compression strength, the compression setting, and the heat conductivityare measured using the same methods as in Example 38, and the resultsare shown in Table 7.

COMPARATIVE EXAMPLE 20

Using shaped foam pieces obtained from Comparative Example 17, a plasticfoam material is obtained in the same manner as in Example 42 exceptthat the shaped foam pieces are distributed at 3420 g/cm².

The expansion ratios of the core material and the cover material makingup the plastic foam material, the bending strength of the plastic foammaterial, the 25% compression strength, the compression setting, and theheat conductivity are measured using the same methods as in Example 38,and the results are shown in Table 7.

Referring to Table 6, the final foam bodies have superior compressionstrength, bending strength. Furthermore, when shaped foam pieces arearranged so that they are positioned at the vertices and centers of thehexagons making up a imaginary honeycomb, further improvements in theplastic foam material's compression strength and bending strength areobtained.

The composition of the present invention is as described above. Theplastic foam material of the present comprises a core material having arelatively high expansion ratio and a cover material having a relativelylow expansion ratio or a non-foaming thermoplastic resin. Each latticeor hexagonal space in the cover material or thermoplastic resin isformed integrally with the core material.

Because the cover material and the thermoplastic resin body havecross-sections shaped as lattices or honeycombs, the plastic foammaterial of the present invention provides superior compressionstrength. The core material is light and has good heat-insulatingproperties. Also, since the cover material and the core material areformed integrally in the present invention, lightness, compressionstrength, heat-insulating properties, as well as resilience are betterthan is possible by using the cover material, the core material, or thethermoplastic resin by themselves.

Also, by using the method for making foam bodies according to thepresent invention, it is possible to produce the foam bodies having theadvantages described above very easily.

The raw material and the reagents used are listed below:

(1) Thermoplastic resins (un-cross-linked)

High density polyethylene 1: manufactured by Mitsubishi PetrochemicalCompany, Limited; commercial name EY40H; density 0.954 g/cm³ ; meltindex 1.5 g per 10 minutes.

High density polyethylene 2: manufactured by Mitsubishi PetrochemicalCompany, Limited; commercial name PY20A; density 0.951 g/cm³ ; meltindex 9 g per 10 minutes.

High density polyethylene 3: manufactured by Idemitsu PetrochemicalCompany, Limited; commercial name 130J; density 0.956 g/cm³ ; melt index11 g per 10 minutes.

High density polyethylene 4: manufactured by Mitsubishi PetrochemicalCompany, Limited; commercial name JX20; density 0.954 g/cm³ ; melt index20 g per 10 minutes.

High density polyethylene 5: manufactured by Mitsubishi PetrochemicalCompany, Limited; commercial name BZ50A; density 0.953 g/cm³ ; meltindex 0.35 g per 10 minutes.

High density polyethylene 6: manufactured by Mitsubishi PetrochemicalCompany, Limited; commercial name HY330B; density 0.952 g/cm³ ; meltindex 0.6 g per 10 minutes.

Polypropylene 1: manufactured by Mitsubishi Petrochemical Company,Limited; commercial name MH6; density 0.90 g/cm³ ; melt index 1.2 g per10 minutes.

Polypropylene 2: manufactured by Mitsubishi Petrochemical Company,Limited; commercial name MA3; density 0.90 g/cm³ ; melt index 11 g per10 minutes.

Polypropylene 3: manufactured by Mitsubishi Petrochemical Company,Limited; commercial name BC5C; density 0.90 g/cm³ ; melt index 2.8 g per10 minutes; containing 810 parts by weight of ethylene component.

Ethylene-vinyl acetate copolymer: manufactured by MitsubishiPetrochemical Company, Limited; commercial name V113K; density 0.925g/cm³ ; melt index 3 g per 10 minutes.

Polystyrene: manufactured by Asahi Kasei Company, Limited; commercialname 681; density 1.05 g/cm³ ; melt index 3 g per 10 minutes.

(2) Silane-modified thermoplastic resins

Silane-modified, cross-linked, polyethylene: manufactured by MitsubishiPetrochemical Company, Limited; commercial name LINKLON HM-600; meltindex 10 g per 10 minutes; gel fraction after cross-linking 60 parts byweight.

Silane-modified, cross-linked, polypropylene: manufactured by MitsubishiPetrochemical Company, Limited; commercial name LINKLON XPM-800HM; meltindex 11 g per 10 minutes; gel fraction after cross-linking 80 parts byweight.

(3) Cross-linking catalyst

Dibutyl tin dilaurate

(4) Foaming Agent

Azodicarbonamide: manufactured by Otsuka Chemical Company, Limited;commercial name SO-20; decomposition temperature 201° C.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing the scope or spirit of the invention as defined in theappended claims.

What is claimed:
 1. A method for preparing a plastic foam materialcomprising thermoplastic resins, comprising the steps of:preparing about100 parts by weight of at least two substantially incompatible resinsselected from the group consisting of a polyethylene, polypropylene,ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, andpolystyrene to form a first mixture; adding from about 1 to about 50parts by weight of a silane-modified thermoplastic resin of the samepolymer type as at least one of said two substantially incompatibleresins to said first mixture; adding from about 0.001 to about 2.5 partsby weight of a cross-linking catalyst effective to cross-link saidsilane-modified thermoplastic resin and; adding from about 1 to about 20parts by weight of a foaming agent to said first mixture to form ablended resin composition; extruding said blended resin composition toform an object; exposing said object to a first cross-linking source toform a cross-linked object wherein said cross-linked object includesonly cross-linked silane-modified thermoplastic resin; and exposing saidobject to a foaming heat source to form said plastic foam material. 2.The method of claim 1, wherein said object is one of a thermoplasticresin sheet material and a thermoplastic resin strand material.
 3. Amethod for preparing a plastic foam material comprising thermoplasticresins, comprising the steps of:preparing from about 100 parts by weightof at least two resins selected from the group consisting ofpolyethylene, polypropylene, and ethylene-propylene copolymer to form afirst mixture; adding from about 1 to about 50 parts by weight of asilane-modified thermoplastic resin of the same polymer type as at leastone of said two resins to said first mixture; adding from about 0.001 toabout 2.5 parts by weight of a cross-linking catalyst effective tocross-link said silane-modified thermoplastic resin; adding from about 1to about 20 parts by weight of a foaming agent to said first mixture toform a blended resin composition; extruding said blended resincomposition to form a thermoplastic object; exposing said thermoplasticobject to a cross-linking source to form a cross-linked thermoplasticobject wherein said cross-linked thermoplastic object includes onlycross-linked silane-modified thermoplastic resin; and exposing saidcross-linked thermoplastic object to a foaming heat source to form aplastic foam material.
 4. The method of claim 3, wherein saidthermoplastic object is one of a thermoplastic resin sheet material anda thermoplastic resin strand material.